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Ur-Ghast's call + Deleb with machines... it's not like the Order factions with their various units; it's even stronger...
When a Nightmare is placed in front of Paladins, and it immediately materializes, it completely negates their momentum... and a Hound is placed next to the Archers. If they are blocked, and they need to be blocked from all sides, then hit them with the Hound and then use mass spells, including the 3 Fireballs from the Ballista... As for Light Magic... I'm not particularly into the Order, but in my opinion, it doesn't have much mana. When I played against it, my Imps handled it quite well. In a very long game, Imps and the Academy can deplete mana (there's never enough mana for any castle, especially against Demons).
The Academy has the same situation: 3 Archers are not only a plus but also a minus in melee combat; they don't shoot very well, and they can't defend themselves effectively.
And various mana-draining spells, like those of the Necromancers, are also good, but correct me if I'm wrong... My Imps essentially force the enemy hero to waste an entire turn. Plus, you can move the unit that the spell is cast on, etc.
Demons are a normal faction; they're just not suited for mid-game. They perform well in fast and long games.
And Banish is useless, in my opinion. It doesn't destroy even half of the gate, and it takes up the hero's turn, while gating doesn't take up the hero's turn. While you're banishing, the Demon hero can use Subdue/Berserk/Bless or simply bombard the blocked Archers with Meteor Shower and Fire Wall.
Demons are strong, especially with their ultimate ability. They may not be stronger than other factions, but they definitely don't lose when properly developed. It's just more difficult to play with them; people are used to the fact that by levels 2-3, some kind of cheat appears for creeping and leveling up... At least it's harder for Warlocks and Demons... And by the way, my opinion is that Warlocks have much better creatures than the Academy (this is in response to someone saying that the Dungeon has weak creatures)