Continuing the request for Army Comparisons (yes, I'll be getting to some of the other popular ones as well) we have Orks vs Tyranids. Both of them are nominally horde armies (but don't really play that way because hordes are kinda awful in 5E) with a good combination of shooting and assault potential. Orks can bring Battlewagons and other armor to the table, while Tyranids will usually be leaning heavily on their T6 critters to even the field for them. Neither side has a clear advantage over the other's primary strategy, so in most cases it ends up being a pretty brutal slugfest no matter who ends up winning- which, from a fluff view, is hardly inappropriate.
This is an interesting matchup in a lot of ways because two very similar armies (melee-focused hordes) going head to head really highlights the differences between the two of them. Both of them have some fairly divergent builds, but in 5th Edition I think there are pretty clear superior versions (albeit with some variation), so those are the ones I'm going to focus on.
Ork Advantages and Strategies
The "main" Ork list is going to look pretty familiar: 3-4 Battlewagons with Deffrollas, 1-2 Big Meks, Lootas in the back, and Buggies or Koptas flanking the main force. This army is relatively fast and resilient and can put out non-insignificant firepower; moreover, it has solutions to many kinds of enemy armies and brings high AVs with cover saves wherever it goes. Given how quickly it can deliver a major assault force to the enemy's face (thanks to open-topped vehicles), it does not take a particularly large number of passed cover saves to see a LOT of Boyz swarming into the middle of an army and wreaking havoc.
The sheer efficiency of mobs of Boyz is one of the strong points of the list- 3-4 attacks on the charge, Fearless, and a hidden Klaw in every squad are all pretty scary stuff and few other armies in the game can match up to that 6pts/model price. While they may suffer a little when receiving rather than delivering charges, not a lot of units in the game want to try and chew through that many bodies in succession, especially not when they are eating up Klaw attacks every turn to boot. Many of the other options in the Ork codex also come in at bargain-discount prices- Rokkit Buggies are some of the cheapest vehicles in the game, Battlewagons cost basically nothing for the AV14 and S10 hits you get, and the Killa Force Field not only fills your HQ slot on the cheap but also brings an incredibly important tool.
Backed up with the excellent torrent shooting of Lootas to disembark the enemy from their metal boxes and Rokkits/Deffrollas as backup plans against various targets and you have a reasonable-enough anti-armor contingent to handle things. Like many melee armies the shooting component is largely only there to enable the melee units- Stunned and Immobilized units are usually quite enough- but when facing a foot army these tools can be used to force the enemy to fight on your terms or cut away problematic targets so the rest of your forces are free to engage as they please. In the Tyranid matchup especially your big guns will find their use in bringing down enemy AT shooting and breaching Gaunt screens to hit the jucier MC targets within.
Orks, being the mechanized list, also have the advantage in mobility- while Tyranids are hardly static, they simply don't have the option to Tank Shock, move 13"+ in one turn, etc, that mech builds can bring to the table. This is also useful in creating terrain on the table when the vehicles are eliminated; Tyranids HATE having to cross through terrain when making assaults, and in this fight getting the right assaults at the right time is absolutely key. Preventing them from striking before you (and thus cutting down the number of attacks you can make) is very key to winning this fight.
Overall the Ork plan is not significantly different than it usually is (get close, krump faces), but Tyranids can bypass several of your usual defenses and bring expendable units that they can sacrifice to make sure that charges go in their favor. Because of this the Ork army will relatively often have to play the defensive role in this matchup, using Lootas, Rokkits, etc, to wear down their MCs in preparation for a game-winning assault. You have superior threat range in most cases (48" off Lootas, 22+d6" charge range from a Battlewagon), so play to the positioning game and try to force them into positions where you can crush them decisively. Shoota Boyz over Sluggas is very useful here- being able to spray shots into their lines gives you an excellent attrition tool and pairs well with the rest of what you will be doing.
Tyranid Advantages and Strategies
Hive Guard. If one unit defines the entirety of this matchup, it is Hive Guard. This isn't surprising, as Tyranid players should be quite used to having their Hive Guard win (or lose) the game for them, but the ability to shrug off the Orks' all-important 4+ cover save is extremely key. Hive Guard will cut down their vehicles, ID any Nobz they happen to bring and just generally ruin the day of any Ork unit they happen to look at. This is good, because many of the other tools they bring can be extremely hazardous to your army, so it is imperative that you find ways to counter them.
Unlike many armies, you aren't particularly scared by the thought of Orks unloading into your lines T2 or T3; in fact, you can only hope that they rush into such an aggressive set of charges, because with a proper deployment your Gaunt screens will prevent them from doing any kind of meaningful damage to you with such an assault and your countercharges can easily break the back of their army in punishment for such rashness. The Orks may bring more sheer combat ability to the table but you, counterintuitively, have the advantage of winning the attrition war thanks to Tervigons and other MCs. Only his Klaws and Lootas represent any threat to such models, so if you can deal with such threats (by pinning them out of range of the MC with multicharges, backfield assaulters, etc) you should be able to win the fights easily and No Retreat! casualties on both sides will ensure that whoever starts to win a fight will quickly finish it as well.
Making good charges is absolutely essential to coming out ahead in this battle, as will careful positioning of your units. Deffrollas make it possible for the Orks to breach your lines, but if you deploy in a sufficiently-tight formation you can make it so that he has no place to disembark his models to- your Gaunts will only move the 1" minimum away from his tank, remember, in most cases leaving no room for his Boyz to disembark inside your defensive ring. Casualty removal will likewise be key in such situations- you may, in face, need to "double wrap" your lines with two units of Termagants to ensure that his opening up with Shootas/etc does not allow him to break through your Termagants and get multi-charges. You MUST avoid multi-charges on MCs and Gaunts at all costs- No Retreat! is a double-edged sword in this matchup, and if you let him take advantage of it it WILL lose you the game.
Many of the MCs also bring some pretty effective anti-infantry guns to the table, from Devourers to Cluster Spines to the various templates. Combined with the fairly-reasonable shooting you can get in abundance from your smaller bugs, this allows you to leverage another major strength of Tyranids- shooting followed by assault tactics, and Orks (being Fearless for the most part) don't have a lot of ways to spoil your plan when you do this.
Since the Orks will outgun you in a pure shooting war (thanks to starting inside tanks and having access to cheap long-range firepower), you're going to have to be on the offensive most of the time. Superior resilience (from expendable units, T6, FNP, or several of these things at once) and initiative values make this a viable plan, but at the same time it requires careful maneuvering to first disembark the enemy and then set up appropriate charges without being forced to cross difficult ground, which can easily spell the end of an assault. Orks' universal T4 and the many bodies shielding their "real" damage-dealers mean that an ill-timed assault can be disastrous, but contrawise a well-executed one can annihilate the enemy with virtually no casualties on your side.
Overall, the Tyranid strategy is going to be one of attrition; where the Orks will start strong and decline over the course of a game, the presence of 2-4 Tervigons means you should be able to continue bulking out your forces over a number of turns, giving you the forces to trade tit-for-tat with them time and time again without seriously degrading your capacity to score objectives, etc. The Ork player will try to outmaneuver you, but given that you plan to be advancing as a singular, unstoppable mass of models this may not be particularly effective against your army. Keep everything together, take advantage of your aura buffs and debuffs and you should be able to put a real hurt on him.
Non-Issues
Now, I only really talked about one type of matchup (Mech Orks vs Nidzilla) in the above section, for a couple reasons. For one, discussing the combinatorials of how six or seven different builds stack up with each other is... well, to be frank, a waste of my time and yours as well. As I said, each of these codices really has one standout build- not LITERALLY the only way to go, but one that is significantly stronger, all around, than the others. But let's take a moment and see how some of the "alternative" builds stack up here as examples of what I mean.
Basically, take the above Ork army and remove the advantages of mobility, durability, and flexibility and you have Green Tide. You know what Tyranids aren't afraid of? A bunch of bodies spread out across half the field, that's what. While it may have way more mans to deal with, Green Tide is largely incapable of bringing them to bear effectively and is very, very vulnerable to being picked apart piecemeal as it struggles to get its more distant mobs into range of an enemy that is chewing its forward elements apart. Tyranids can quite reasonably shoot down any minior vehicle elements early on, use Catalyst to protect one or another squad from the worst of the damage and then multicharge its entire army into one (or two, depending on how sassy it feels) mobs of Boyz, walloping them with casualties and then doubling up thanks to No Retreat! With any surviving Boyz likely blocking easy charges and the potential to consolidate behind screens if the enemy is wiped out, Green Tide's only real plan is to throw more meat into the grinder in hopes of clogging it up, which is not exactly a highbrow or effective plan.
Reserve Tyranids, on the other hand, suffer from the exact opposite problem: as a horde army that relies on overwhelming the enemy, they... don't bring all that many bodies and pretty much nothing they do can be classed as "overwhelming." Zoanthropes are okay for breaking some tanks, but the Orks retain their 4+ cover in this case and you're generally paying 50% more than you would for Hive Guard (due to needing a Spore), so you almost always take some major hits in the AT department. Moreover, Buggies will be a serious pain and can slap down Zoanthropes with annoying efficiency, not to mention punishing the Tyrant, Trygons, etc. Devilgaunts, Ymgarl, and other staples of the reserve list can likewise annoy, but not seriously disrupt the Ork army, as they just don't have the sort of large-scale firepower that is needed to really cripple it. (Remember, even 20 Devilgaunts will only kill ~8 Orks before dying to the return shooting and assault, even assuming they are out of their transport.)
There are few armies in the game better-suited to ruining the ubiquitous Kan Wall than Tyranids, when it comes down to it. Hive Guard can punch holes in their AV11 with ease and backup assaults from monstrous creatures can likewise cause devastating damage before the Kans get to swing- not to mention only being hit on 5s, etc. And while the Boyz advancing behind the wall are often just as scary to many armies, as we've already discussed above Tyranids have little to fear from such a force, naturally equipped as they are to deal with most infantry targets quite efficiently. Bar a ridiculous string of 1s and 2s on the damage table, Tyranids should wipe a Kan Wall off the table quite effectively.
Stealer Shock armies gained a lot of popularity during 4th Edition and have continued to see play in 5E despite lacking any particularly good ways to deal with most types of armies. Against Orks they will struggle to deal with the vehicles that should be roaming around the field in abundance, but also suffer by their very nature as anti-MEQ specialists when fighting mobs of Boyz. While causing 6-12 wounds (including some Rends) may be fairly threatening to many armies, Orks can soak such casualties with relative ease and Genestealers are far, far too fragile to shrug off the torrent of attacks that will be delivered in return. Against a foot-based Ork army they can use their superior speed and deployment options to try and get the pounce on them, using the usual Tyranid plan of concentrated force and multicharges to take advantage of No Retreat! to maximize damage, but a mechanized Ork list (or one that runs any other kinds of vehicles, including Trukks, Kans, etc) will give them a very bad time.
Nob Bikerz are so long past their expiration date at this point it's rather sad, especially since they were once considered supremely overpowered and nigh-unbeatable. (Of course, this was just as dumb as most of the internet's other suppositions about 40K, but...) In the modern day they are rare but still seen occasionally, either as part of a broader force or by someone still latched onto the strategies of half a decade ago. Hive Guard alone can cut such an army to ribbons in short order, ignoring their FNP, multiple wounds, and various cover saves and leaving them only with a desperate roll of their 5+ invulnerable in hopes to survive. Combined with Gaunt screens to stall early assaults and combined Gaunt/MC assaults to drag the Warboss or other threats down the army makes an even sadder performance than Kan Wall, if that's even possible.
Overall
If we stick to the "real" matchup of Battlewagons vs Nidzilla we have a fairly hard-fought battle, but probably one that the Orks edge out in (if only slightly.) Excellent mobility from mech combined with good assault potential and backfield shooting that can be hard for the short-ranged Tyranids to deal with mean that the bugs start on the back foot from turn 1. It's quite possible for them to turn the match around with some good movement and terrain, especially BLOS pieces that can limit where the all-too-static Lootas can expect to deploy, will play a large part in how things throw down. However, at the end of the day the Orks, properly built, bring a lot of the right tools to the field and 'Nids are simply too reliant on getting those side shots followed by favorable charges.
