Psions run into different problems, depending on level.
At low level your character is weak. Your defensive powers are rarely as good as a sorcerer's defensive spells, and your offensive powers (especially direct damage) are weak.
Consider: your Ectoplasmic Cocoon power (2nd-level) is better in every way than Hold Person, but it's worse than Web. There is no psionic web. Many disciplines don't have
any good 2nd-level powers, and some disciplines are much weaker than others.
At high level your character is weak. You can now use the better powers in your disciplines. For instance, a savant could give up on direct damage and just use Disintegrate, or you could use the Mind's Eye (in my sig) and try to twink it. You could do 26d6 damage with True Concussion at 16th-level, but you need a Torc of Psionic Might and the Empowered Psicrystal (Discipline Focus) feat as well as Fortify Power and Overpower to do it, and you can still only hurt one person with that power.
The problem is Multiple Ability Dependency. If that psion doesn't have True Seeing for instance (requires Wisdom 15), then they're in trouble. Then again, they're in trouble if they don't have Greater Energy Barrier (requires Strength 17, in the Mind's Eye). In fact, unless you have a 19 in five of your ability scores (I wish I was kidding) then your psion is severely underpowered.
Or picture an egoist (the warrior with d4 hp/level) without Ablating or Metaphysical Body (Met Body is basically Magic Fang). Your character needs an Intelligence score of 15 simply to function.
(Using both is probably too much of a power increase.)
I disagree, but it does mean some house ruling.
For instance, Mindscapes (aka ITCK 2) has a prestige class called the Lucid Cenobite, a psi-monk. In order to be effective, you have to use the Metaphysical Body chain powers from the Mind's Eye.0
Do the math it works out to roughly the same amount of powers per level a day that a sor gets castable per day.
It's been done, many times. A sorcerer gets 400+ power point equivalents at 20th-level versus the psion's 183, and the sorcerer gets more from a high key ability score than a psion.
Addendum: psions have some broken combos available to them, just like any other class. If you want details about broken combos (preferably to remove them from your game), please ask.