I think that is a better criteria than over/under powered, which are really party and DM dependent.
The issue with the sorcerer (and the berserker barbarian as the other main example) is that to be effective with them as written, you really need to play a cold, strategic style, which is the opposite of the fluff/theme of the class. The disconnect is irritating--there should be more reward for playing the wizard strategically than for playing the sorcerer.
I agree the third casters and the third martials have problems. Largely because people assume that a third caster really means a half caster and 2/3rds melee type (or vice versa for third martials). Usually I am pro-subclass, but I think the thirds would have been better as half caster classes (maybe one arcane half caster with subclasses based on the bard, wizard, sorcerer, and warlock).