...I'm curious if this is another example of something that has far fewer variables than people seem to think. Imm spoon?

Sure is.

There's your skill... I think there's probably an edge that helps a little... and wight/mummy/lich-level necromancers get a small bonus too.

And as far as I can think of off the top of my head, that's it.

An edge to aid in raising zombies in graveyard-flagged rooms would make sense. Though, necromancers have plenty of advantages in these places already.

What I'm fairly sure of: Animate dead suffers a penalty to success in daylight conditions.

Other assertions I or Quix made on Dios:
Moon phase has an effect on success.
Corpse level vs cast level effects success rate.
Whether or not corpse is embalmed effects success rate.

I'm positive none of these is a factor.