Having played a bunch of burn teams, I think Demon might be a tad bit overtuned. While it's fine at lower levels, the fact it gets 4x multistrike + native burn + free attack (when maxed out) means its burn application entirely outpaces every other burn unit in the game, even legendary ones, it's not even close. By comparison, the other epic burn unit (the one that consumes burn to deal extra damage on attack) is so bad that you'll never take it. The damage it deals is almost never worth it, as any significant enough amount of burn will just kill your target anyway. It doesn't help its HP is lackluster too, which imo doesn't fit an epic unit at all.
I haven't tried other teams enough to get a solid understanding of the balance of them, but I like the game thus far, even if I'm not a huge fan of units that do nothing if you don't encounter a certain enemy type (like the one that gets buffed on breaking enemy block) as your opponents are entirely random and it makes strategizing and using them hard, more flexible units that can counter multiple archetype (like the cleansing dragon) don't have this issue as much since they counter more types of team.