Will some classes turn out to be inherently overpowered?

Classes in MMOs can be designed in such a way that they are impossible to balance without breaking the whole concept.

An example is the cleric of Vanguard who fights in heavy armor and heals efficiently. It was a worthy goal to try to break the healbot stereotype but other melee classes have no chance against the cleric in 1v1 PvP in the game.

I see similar dangers in AoC as my understanding is that the classes will have similar damage output. The tank/healer classes may be overpowered.

There will always be fluctuations regarding balance, this is just something we have to accept. But we are trying very hard to make sure that all classes meet their expected levels of output etc.