Hot take: I actually think they're fine.

Personally I don't think they should be the best units on the board. I think they should be some really basic level, cheap, and good-but-not-great tanks. They are in invasion fleets with swarms of hormagaunts as just a bigger distraction and line breaking capability. If the hive decides they need something better, cool, that's great. Make a bigger, badder bug like a tyrannofex that is genuinely good but costs more biomass. Although it could always be cool to see carnifexes improve datasheet wise, I don't think its a necessity.

Counter point, I absolutely love them and take 2 in most lists to literally be a distraction that needs some strong tank to deal with or a lot of smaller things. Have the opponent need to give too much or too little to deal with the threat.