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Are they gonna be worth it in 5th edition, with the running option and the more infantry focused rules?
Possibly. The pseudo fleet rule may help them get into melee much faster. We'll have to see when the new book comes out. It's worth testing.
I think assault-type Carnifexes may be much better now. With the running rules they won't be that hard to get into assault. Also because they are moving faster they will be much more of a threat to an opponent. Many people were able to happily ignore Carnifexes with close combat biomorphs because they knew the Carnifexes had little chance of reaching them. If they can move faster they will be much more of a problem as they will pose a huge threat to tanks and other big types. Basically people won't have the option of ignoring them anymore.
Also I heard that infantry squads can't shoot through each other. This may mean that spped-bump units such as Kroot won't be as handy anymore. Without these things in the way we will have an easier time stomping all over people in assault. Also it may reduce the amount of firepower thrown in our faces. I think that because the new rules indicate they are more infantry based that this will be good for Carnifexes. A Carnifex is a monstrous creature and monstrous creatures still follow much of the basic infantry rules at the end of the day. This is similar to how gargantuan creatures may be tank flattening giant beasties but they still use many of the basic rules of infantry.
From what I've heard, monstrous creatures will only be able to fire two weapons if they hold still. They can still move and fire one. I think we'll be seeing less nidzilla lists if that's the case. Also, the run rule is rumored to work similarly to the fantasy version, in which you can't run if there's an enemy within 12". That makes spore mines that missed quite valuable (plus no more partials, welcome back Biovores).
In all, I think the Carnifex will have a new role come 5th. I can see armies using an anvil/hammer approach with the Fexes on the Anvil side.
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The monstrous creature 2 only if still rule is wishful thinking ... hasn't been confirmed anywhere, and isn't in the leak that I've seen. AFAIK they can still move 6 fire 2.
Running is all well and good, but Fexes still blow in close combat even with all their upgrades, except against certain opponents. Also, you can still gank 'em, especially when they get close ... meltaguns and plasmaguns and rapidfiring popguns.
Nids & Guard
GMail = MVBrandt
you have to remember that infantry can still fire at monstrous creatures and tanks over interveneing infantry still. the only way infantry can't see them is terrain or other large models. So when units can't see anything else because models are in the way, the carnifex will see more shots coming its way.
The new vehicle damage chart makes venom cannons way less effective, so I bet we will see more combat carnifexes. Or else just BS fexes, since you don't need to roll to hit anymore.
--Famous last words of Asmodean
Wait for the FAQs being released alongside the new rulebook. It's at least a possibility that Venom Cannons will become AP- instead of "can't penetrate."
AP- can penetrate under the new rules, at a -1 modifier. As a result, penetrating VC hits would still kill vehicles on a roll of 6.
Nids & Guard
GMail = MVBrandt
And how do I wish you're right, MVBrandt. Certainly it's the best solution, either that or simply saying "they have a -1 in the vehicle damage roll" instead of "they can't penetrate" (Which would be similar to changing the AP but would keep it's AP vs infantry).
I'm really curious about how they're going to do the update (rumours mentioned that it would be sold as update booklets instead of FAQ).
Anyway, even if they could only shoot a weapon while moving, my carnifex tactics wouldn't change much, move somewhere within 36'' while blasting away and sit in cover firing all day long. Elite dakka-fexes would be hurt, though, but we still have to see what the rule will be.
And (yet again) MVBrandt makes a good point about the melee carnifices. They have very low I, and they can't compete with dedicated melee units. Too many things can kill it before it gets to swing. (Hidden powerfists, powerswords, GKT nemesis weapons, rending, the nightbringer, daemon princes, etc.)
That said, I love fielding my "so ugly its cute" paperweight from time to time, even with 4e rules where he can't run. He doesn't do very well, but I do really have fun! (And I still remember that day when I got to charge some banshees! my friend learned that day, however, and it's never happened again).
Yeah, a CC fex will pretty much always remain a "one trick pony". You'll use it to awesome effect once. Than your opponent learns to not get into melee with it unless he can win. After that, it's pretty much the "special needs" kid on the map. "oh, look at the little guy, trying to get into range to fight. He's so cute!"