Dross
Explorer
I think some of the problems are situational or a matter of style (only play 3.5 so can't comment on 4).
Psionics are "more balanced" if you have 4+ encounters a day, denying the supernova phenomenon. If your style has less then 4, then the discrepancy is noticeable. At least this is what I've been told, since we don't use them much.
A hybrid character can perform to the best of their abilities in more situations that a strait class character. While not as strong as the strait classed character for performing a given role the wider "usefulness" can make it seem more powerful.
Psionics are "more balanced" if you have 4+ encounters a day, denying the supernova phenomenon. If your style has less then 4, then the discrepancy is noticeable. At least this is what I've been told, since we don't use them much.
A hybrid character can perform to the best of their abilities in more situations that a strait class character. While not as strong as the strait classed character for performing a given role the wider "usefulness" can make it seem more powerful.

. I think with hybrids, it's the same issue I've seen with psionics, incarnum, and even Tome of Magic classes (That's right, I've heard that Truenamers are overpowered) - "the only reason you'd use a system the DM doesn't know is because you have some kind of mad exploits in mind."