I've always been a mage in my mabi character, and I've learned that... mages are slow.
Yes, they can hit hard, but you will need some time to prepare.
Take hailstorm as an example. Oh yes, you can hit a massive 30k, but for that you have to pass throught 10 seconds of cast time, and other 3~ seconds for full 5 charges of ice. If you don't have time to prepare, or doing a mission that spawns a lot of monsters, you won't do shit.
It's play style is not dynamic as melee, fighter or what else, as most of the time you will be sitting there trying to cast your charges, when everyone else is just jumping and doing stuff.
Damage-wise, you can hit pretty well, and pretty fast if you can afford getting casting speed reforges (what is very costy).
But all falls to where you will be sitting there casting and casting. And if you ever get multi-aggroed, and you don't have access to any other style or snap cast, you are pretty well doomed.