I find myself checking the surrounding provinces for shops now before deciding what to build first. If there is a leather/archery store in your home province or right next to it, it's probably worth while to go with a scout and build around that. Likewise for plate/warrior or knowledge/wizards.
Though I was very disappointed in the demonology spells so far. Fiends are really fucking weak units for a tier 2, and dark pact on a tier 2 unit only makes a fiend. I guess it's a super late game thing, where the idea isn't so much that you're getting an awesome unit you couldn't get otherwise, but rather that you're not really losing a unit if you let your tier 3/4 unit die, just trading it in for the demon version.
Ghouls are fucking amazingly powerful though. Once I had enough slots for 3 of those I didn't even need zombies any more. At decent levels they disease, poison, and ruin morale. Disease rapes. I can't imagine what higher tier necromancy units can do. And ghouls have 2 movement, so you aren't stuck with a 1 mobility army.
Yes, necromancy seems to be way better than demonology from my observations. Imps are a joke due to their hp. Hounds arent bad but only comparable to harpies in level two tier. Fiends arent worth mentioning as you said, nothing really goes for them. Demons just suck. Havent seen a devil so cant comment on his stat but he should be a level 4 unit with all included perks.
Undead on the other hand are very good. Skeletons have nice ranged defence and pack quite a punch, zombies are slow and tanky, but man ghouls are indeed amazing. But ghosts are just crazy, their defensive stats make them almost invulnerable to harm. My level 15 warriors couldnt beat one randomly found in a crypt (physical damage does almost nothing). Then there are vampires that I havent seen but I imagine arent bad either. Im toying with the idea of playing a dedicated necromancer for once but would need raise ghoul spell for that.