Magic in Heroes of Might and Magic V.

Mages can counter any magic with a counterspell, no matter who casts it. And if they manage to get a leadership skill, they become incredibly powerful.

I wouldn't say they're overpowered. Because mages have problems with physical damage. No matter how good their mini-artifacts and artifacts are, they can't match regular warrior heroes. Therefore, trading our hero for an enemy hero is not the best option. Unless, of course, it's a "mage vs. mage" battle.