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

I think, on the contrary, that it’s easier to play against the Orcs. Firstly, Nightmares usually attack first, and the attack can be directed at the Centaurs. Then Cerberuses and Greater Demons join in. At the very beginning, the Barbarian units are unlikely to have many points of Blood Wrath (except for Pure Rage). Secondly, Temptresses can charm Cyclops or some other powerful unit. And Demon heroes often get Dark Magic.