I've seen a lot of funny comments around the Intertubes which is suggesting the relationship (and who's wearing the pants) between shooting & assault isn't really understood. Particularly given the transition we've gone/are going through. This relationship is pretty much the same as what it was in 5th edition and this is what's sparking the confusion bubbles over my head in very cartoon like fashion. Enough preamble - let's decide who's wearing the pants!
Shooting -
In 5th edition cover was easy to get and a universal 4+. This made shooting at things hard to remove but kept your heavy weapons alive. Wound allocation through torrent of fire could see them disappear however. With 6th edition, cover is harder to get for basic infantry and generally lower at a 5+. It's easier to get for vehicles and monstrous creatures however and ruins + fortifications (which you can buy) provide equal or better covers compared to before. Add in that you can manufacture squad placement to ensure your heavy weapons are the last things to die unless you get flanked (and thus likely have other issues) so the reduced cover in these terms is somewhat mitigated.
So like in 5th edition, in 6th edition torrent of fire is the way forward with shooting. You need lots of guns and lots of shots and they will do damage. On top of this - there are a very wide range of weapons which allow you to ensure the shooting does what it needs to do and most units are capable of bringing some form of duality to the table where they can affect more than one unit type efficiently.
Shooting also happens from further away than assaults and when I shoot you - you don't get to return fire against me (waits for unit which can now do this; no I'm not including the TWO! reflection abilities out there currently). It's simply a one-way street for the most part - I shoot you and you either die or live. This is has led to the fallacy of course that whoever goes first wins as they get to shoot first. We've hopefully debunked this and it should hold true for this edition, too. This makes shooting harder to stop - you can't block it easily or make the unit go around an obstacle and basically remove it that turn from the equation - it can still maneuver around it and...shoot! With deployments also putting armies at a minimum 24" apart - and two-thirds of standard deployments now allowing much greater distances to be achieved, shooting has that extra bit of wiggle room in terms of keeping assault options at arms bay.
In essence, shooting is about winning the game while you maneuver to keep doing it. And it's reliable and effective.
Assault -
Assault is just as deadly in 6th edition as it was in 5th edition if you're looking to wipe out units. With a huge reduction to Leadership if you lose and whole units being swept away in sweeping advances, losing combat is a very dicey proposition. However, with new Fearless rules and Allies to spread ATSKNF + the usual stubborn goodness abound, getting those sweeping attacks off is harder or sometimes impossible. If you want to kill a unit though - close combat can do a very good job of it as most models have a greater damage potential in combat (particularly on the charge) than via shooting. There are obvious exceptions to this...
The run rule introduced in 5th edition is a mechanic which helps assault units get to where they need to go faster. This hasn't changed but the disembarkation and cover changes have affected their ability to get into combat. With cover reductions assault units can be a bit weaker by the time they get to the opponent, particularly with poor save units like Hormagaunts or Orks - less so for tougher units like Paladins obviously or units which generate their own cover (Bikes). The Run rule still helps here, as well as any other forms of movement you can get in the shooting phase (i.e. Turbo-boosts). Which of course ties into...
Random Charge Length. Previously the best you could do was 6" unless you were Cavalry or Beasts - then you could go 12". Now everyone can go 12" though it's unlikely, even with Fleet. Statistically speaking the most common results are going to be between 6-8" though in terms of reliability, the 3-4" range through open ground and terrain is where most charges will go off (i.e. 90% probabilities). Either way, longer charges are going to happen. Even those extra couple of inches is going to cause more combats to happen even with the advent of Overwatch trying to stop such from happening. And some units just couldn't give amusing object here about Overwatch and thus can really benefit from the increased charges RCL brings in - especially if they have Fleet.
Bringing it all Together -
Shooting got better - cover is worse, torrenting is still the king of the hill here though AP2 weapons are now more important and will see more use (thus making high armor save assault units also rue the decreased cover saves) and there is greater capacity to move and shoot. Assault units are more vulnerable because of this but they can charge from greater distances - it's just a matter of getting them in a position where they can (i.e. not in a transport and alive). Their transports can also move faster than before to get them into advantageous positions but you can no longer assault when coming on from the board. This puts onus on having a unit which can survive at least one round of shooting before assault or which doesn't require a non-assault transport. New combat abilities like Hammer of Wrath and ability to eliminate an opponent but force combat to continue without them getting to attack improves the damage capacity of assault units. Challenges as well can be used to snipe characters or force them out of fights, etc. who are otherwise very protected in combats.
The main issue here, and has been for an edition now, is unless you stay in combat for your opponent's shooting phase, the combat unit takes a bunch of close range firepower to the face as you cannot consolidate into a new combat. With the way rallying works as well - the squad you just half destroyed can turn around and still shoot you in the face. The issue here, and has been, you want to limit your damage the turn you engage in combat when that's your most damaging turn more often than not. Against a tough as nails unit, ya go for it, but against a unit you're going to win against you want to win small the first round and crush them the second round. This was much easier in 5th edition and still not a reliable thing. With RCL this is harder to due to the randomness - you don't know how far you are going to assault and with Overwatch, stringing out your squads is riskier. Yes terrain in 5th force this upon you but could get one model within an inch and a minimum of two others in max assault range (i.e. 6") and thus still pull punches. You could also decide to multi-assault after the fact, etc.
A lot of stuff which isn't covered here though is defense against assaults. It's a lot easier to defend against assaults with options like blocking, bubble-wrap or by simply denying super strong units the ability to wipe out multiple units (space them out) and forcing massive overkill. You don't really care if you lose one tiny squad of plebs or a cheap transport to the 700 point deathstar if the rest of your army is functioning smoothly - particularly since that rest of the army can now lamb-blast said deathstar if it wants or it can just continue using delaying tactics. It's a lot harder to defend against shooting because it happens at a distance - you can mitigate its damage through cover or killing it yourself but stopping entire units from shooting at an entire portion of your army is very hard to do on a proper board (unless you're playing against a static gunline of course).
Conclusion
All in all, assault is the weaker option compared to shooting despite assault really getting a bit better in 6th edition - longer charges are possible (and if you consider most charges are roughly the same as they were through terrain previously, there is more randomness here but more gains to that randomness than losses), new rules like Hammer of Wrath, vehicles much less durable in combat, etc. it's not always about killing as much as you can. This is very much the case with most combat options and whilst they can tear through units - its the aftermath which hurts them the most. Basically, you MUST bring shooting to the table to enable your assaults; the reverse is not true.
Now this isn't to say assault options are bad or it's a useless concept within the game. VERY far from it. Rather, an army composed entirely or built around, assault options is going to struggle in some ways compared to an army built around shooting. That's not to say primarily shooting lists always win. They didn't in 5th and are unlikely to in 6th. Even one combat unit can be a scary proposition for the opponent if it breaks into their lines - it may die but they have to deal with it whilst other elements continue shooting it. Even beyond this, aggressive armies like Space Wolves or Jumper Blood Angels can still get lots of mileage on the tabletop.
