So I got to talking with my friend last night over the new 'Nid 'Dex and I got to thinking that the Carnifex spam is looking like a viable option. While it's apparently not as tough as 4th, I get a feeling that if you put 9 Carnifexes with regeneration in Heavy Support and ran them in a "Shield Wall" style formation backed up by Venomthropes to give them a 5+ cover ("But feels like an Invulnerable") save, then follow it by your little gribblies and synapse beasts then you've got a fairly viable combination going.
My thinking:
CCCCCCCCC
..V....V....V..
TTTT.S.TTTT
TTTT....TTTT
TTTT.S.TTTT
TTTT....TTTT
C = Carnifex
V = Venomthrope
T = Termagaunt
S = Synapse
. = open space
Anyone who wants to shoot at the Venomthropes are going to need to get past a 2+ cover save (+2 for Shrouded and +5 cover for being behind another unit) and everything else is getting a 5+ cover save except maybe the back synapse creature. Everything running forwards could do a hell of a lot of damage even with taking a few casualties in the run up to the enemy lines.
Sure there are a few weakness's to it, you wouldn't want to run into any demolisher cannons in a shady back ally, who would? But you'd still get the cover save. Tau would give you a whole lot of problems ignoring your cover, so that's a problem. The list isn't terribly practical running up to about 1300 points for a full compliment of Carnifexes with regeneration and Venomthropes. So you'll probably need a 2000 point game to get it up and running.
So what say you Hive Mind?