An all mage team or all archer team would be destroyed quickly. The cannon fodder enemies will pose no threat, but boss creatures would destroy you quickly because your defense will be weak and you can't nuke them since most bosses will have multiple health bars. You need a tank in your group to draw creatures attention so your mage/archers have time to do their thing.
An all warrior team would not work as well, because then they would have no rang attacks, thus flying enemies like harpies would be tough to fight. You need a balanced team to do good in this game.
It depends what skills there are in the game, and what the mages could do together, different classes have different assistance skills, though I have a feeling they will probably die quick against bosses, I'm not sure how they will balance that.
We've yet to see what mages can do if they exploit enemies weaknesses, for example for the small enemies that's weak against fire, some of them will roll on the ground if you burn them, there might be ways to stun bosses with spells like melee characters could when they poke a cyclops in the eye. Perhaps darkness or light spells could blind them.
Warrior has a skill that allows you to launch other characters up in the air, so does fighter, so maybe there are more we haven't seen.