I wouldn't call them "OP" as each transformation has drawbacks.
All:
They're only good for PvE. Because of their CD, you can only use your transformation once in, say, Budokai. Granted it's a great game changer, but still something you need to use wisely.
SSJ: As people said, Fighters and Swordsman can't hold the form for long because they have a low ENG stat. EP Regen is also not going to help in the long run because it gets significantly lower during combat. Even Turtles and Cranes can have a hard time using it if they're caught in combat for a long time. Then there's that 15 sec stun that happens when the form shuts off.
Great Namek: Aside from the obvious SP requirement for their skills, the only thing Great Nameks suffer from is their 10 minute duration. Dragon Clans are more suited for using them because they have more power in their transformation skills than the Warrior class, but again you're forced to invest at least 2 SP (one for each skill) just to use them. Otherwise, you're stuck with basic attack and one skill that you can use from your normal skillset. Oh, and their movement ability sucks.
Pure Majin: Basically the combination of SSJ and Great Namek: drains your EP and you're forced to invest SP in the skills. Wonder Majins are really the only ones who should use Pure Majin because they aren't needed in their normal form as much as Mighty Majins (aka your buffers). The skills for Mighty Majins are also weaker than Wonder Majin. Oh, and they can't dash.