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

Dark Elves have little mana, and if Imps move quickly and steal it, it may turn out that Demons have a better chance.

You're not entirely right. Playing against Dwarves isn't that difficult, as Dwarves have low damage, attack, and initiative. So, Demons have time to summon reinforcements without significant losses.