Against which factions is it easier for demons to fight, and against which is it more difficult?

I would agree that if a demon doesn't have Sandro's cloak in the late game, it has no other choice but to go for light. Imps are indeed one of the biggest headaches for a Necromancer. But I think a smart Necromancer will focus on knowledge against a demon, so they don't run out of mana. And most importantly, you need to use spiritual connection wisely. Regarding Spell Power. I wouldn't say that demons waste it, as even with a knight's set, it greatly affects their attack damage, and thus reduces the number of repeated turns. If Spell Power is available, I would definitely take Deidre. However, on maps without a mentor, leveling up Spell Power is very risky.
Regarding tactics, it's been correctly noted - I always try to use it, but the thing is, there's one demon who negates this ability. In that case, you need to try to defeat the demon with strong units, such as Lucretia and Orson.