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

Against the Elves, it's impossible to play as Demons, even if you disable their archers; their army is still strong and fast, and they easily overwhelm the Demons before they can set up defenses. It's also difficult to play against Orcs. Deleb's crossbow breaks too quickly due to the preparation phase. It's practically impossible to defeat a Gnome Mage if he has the Blood Dragon Potion rune, the resurrection rune, and chaos magic; there's nothing you can do to stop him. However, Necropolis is an easy opponent. They are too slow, and they won't be able to cast spells at the beginning (they won't have enough mana), and later, hypnotizing one unit won't be enough to help against the summoned hordes.