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

Demons perform well against Warlocks because imps drain all their mana (since Warlocks have a weak army, and without mana, the hero becomes useless). However, they perform poorly against Wizards. If you also upgrade "spell mastery," you can banish summoned creatures (the wizard's mana will run out if you don't kill the imps within 3-4 turns). I've already tested this!