The Scarab farm was all about taking tiny amounts of Scarabs and turning them into huge units with nine Spyders. This provided really large Scarab units which could molest even moving vehicles and leave points free for other Necron goodies. In 5th edition it had a few flaws though - Scarabs would hit vehicles, drop them to paper thin armor, destroy them and then turn around and die. Scarabs were a great unit when supported by extra firepower but by taking nine Spyders, one would often lose a key support unit in the Annihilation Barge. Imothek was also common with Farm lists which meant Necron shooting was impacted negatively as well - what are Necrons good at again?
Right so let's look at what changes we have specifically for Scarab Farm as we've looked at the general Necron stuff already...
- vehicles are hit on a 3+ at worst. Scarabs chew through vehicles now. Think the Mummy series and imagine those human beings are tanks and those scarabs are...soulless scarabs. Ya, vehicles hate combat now and Scarabs are one of the best at it.
- Fearless rocks - Fearless sucked before for Scarabs as they would often lose combat and then lose several more bases for their efforts. Anything which could ID them would wipe out entire farms in a matter of seconds but now, well now they are a great tarpit. They aren't going to kill infantry still but they can at least stick around and annoy the crap out of them.
- movement & charge changes make Scarabs faster over a period of turns thanks to a 12" move plus RCL (Random Charge Length) but in a given turn, RCL reliability can bite Scarabs on the bottom. They generally aren't engaging infantry units though so Overwatch isn't that scary despite their poor statline.
- Cover changes hurt Scarabs a lot - they used to be cheap with an annoying durability but now getting such a good cover save is harder. Still possible but harder.
- And Night Fight is no longer what it was. Although most armies generally had a counter to Night Fight through things like Searchlights, it's a lot easier to completely IGNORE Night Fight on some very specific units. You know, like a building Scarab farm which coupled with the above change, makes growing the farm a lot harder to do.
- Spyders are scarier with RCL however since they got potential movement gains though cover reduction hurts them as well - they were pretty easy to get a 4+ for before base on 50%.
And then of course you have all the awesome changes for Necrons in general with Gauss & Tesla loving Hull Points, fake AV13, Jink saves, etc. So what's this mean for Scarab farm?
WHY ARE YOU DOING IT?
Scarabs, particularly lots of Scarabs, were really done as a 'solution' for anti-tank I feel. Imotekh + Scarabs was a pretty good solution for MSU mech in terms of the vehicles for Necrons but it suffered versus infantry. Now Necrons don't need help against mech thanks to Hull Points and their excellent shooting so the question really becomes: do you still want to take away from that awesome shooting available to Necrons and replace them with MCs and Scarabs? The answer should really be no in terms of an entire list construct but you can certainly have a sort of mini-Farm within a balanced list and do well. Especially if you're playing double FoC as you're not really taking away from your ability to take Annihilation Barges.
So if you were really looking to add something like this to your army, I'd be looking at keeping it as simple as possible. Scarabs have some pretty nice upsides in 6th edition and make a nice tarpit unit for your normal Necrons but if you go for a full farm you're losing a lot of extra firepower by not taking the Barges. Rather than consider this an army template, I'd more think of it as a plug-in for a basic Necron army. Hell, it could even be used as an Ally plug-in though you'd obviously only have one Fast Attack unit.
3x Spyders, 1 w/Gloom Prism
Totals: 395 points
You don't want Imotekh as he stops your own shooting. You don't want mass Spyders as that takes away from Annihilation Barges. You want two Scarabs so you have a 3rd Fast Attack slot for Wraiths if you want and you have a second layer of Scarabs. One works as well though, particularly if you're running it as an Ally plug-in. You're not fiddling around with all the extra goodies Necrons can get like a Writhing Worldscape C'Tan + Tremorteks (Crypteks with Tremor staves) because that takes points away from things which actually drop crap dead in the Necron codex. It's a simple mini-farm which can plug into lists if you're looking for a screening option and back-up MCs all for less than 400 points. Hell, the Spyders aren't even needed - I'd still rather another Annihilation Barge and let the Scarabs do their thing - pop some vehicles, delay the opponent.