Honestly zumi is pretty well rounded/OP in a lot of different classes.
For defensive tree:
Their auto regen skill is pretty dam useful, they excel in Templar more than other defensive classes. They get a 15% mace passive and a OP racial, can't really beat that.
They make really good mages (30% eva) or even other general MDPS b/c they have two OP racials. One that turns them into a box like Solid Snake. Basically nulls all PDPS damage and laugh. And ofc the fire push-back racial, very good as well
Ofc zumi MA, injection + charge = stupid capping spd and damage. I think Zumi samurai's are pretty nice too, but Samu is pretty outclassed. Also another escape dash in their racial, so errr...yeah zumi pretty OP here
Mmmm...this is perhaps zumi's weakest section in comparison to their other class. Their racial is nice, but they don't get nice passive to go along with it. But they aren't terrible at it.
TL:DR - Templar, MA, Mage - those three classes...