Well, for 6 refresh you coould purchase Potent Spell-Like Ability 5 times. With Superb Conviction and Lore plus Great Discipline you could drop 15 shifts of power for one stress and one backlash. Attacks would be fine since you only have 5 accuracy. But if you can get 5 fragile aspects or an 11 shift Orbius-equivalent that lasts for 5 turns, you can end most fights instantly.
You'd have weaknesses, of course. You spent 6 refresh on one trick. But when that trick is useful, you'll just crush your opposition. When it's not, you'll probably die.
That's a problem, in my view. Sure, it's easily solved by not building characters like that. But a custom power has to be balanced against the worst case.