The perfect Necropolis.

Hmm. Yes. But the undead don't have children. They appear through killing. Therefore, there is no necromancy. The only thing that could justify the absence of these zombie-griffons in the army is their uselessness. Necromancers simply don't need griffons, cyclops, or any other heresy. (in their dead form, of course). The rest, which are quite useful, can be made neutral units.
P.S.: Just in case, I'll say that the ghouls I wrote about should behave like zombies, and not like what was in Heroes 6.