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

Perhaps I'm missing some nuances, and do the demons have a trump card against the elves?

There's one trick. Take Grok and the new Archdevils. Grok's first spell teleports the Archdevils to the center of the map. The teleport increases their initiative, and they immediately pull the ranged units towards them. This definitely levels the playing field, unless the Haven player is using Wingael.