The warcoven if pretty underwhelming, even though it balances out by the fact that it is purely awesome fluff-wise. I make a point of fielding them every apoc game I play.
I can give you one advice, wich will make your army next to invincible : Fateweaver and Chaos altar. Rerollable 3+ invul saves = win. I have had some people call me a power gamer for this tough : /
"And what are the achievements of your fragile Imperium? It is a corpse rotting slowly from within while maggots writhe in its belly. It was built with the toil of heroes and giants, and now it is inhabited by frightened weaklings to whom the glories of those times are half-forgotten legends. I have forgotten nothing, and my wisdom has expanded far beyond mere mortal frailties."