Psionic is, indeed, powerfull if your games only have 1, 2 encounters. Meaning that a Psionic can blow out all of his Psionic Points in thoses encounters. There is some builds that allow you to do absurd amounts of damage and effects at the cost of wasting /ALL/ your Psionic Points. That’s called ‘Going Nova’.
However, when you start reaching 4 encounters or so per day(or without rest), a Psionic, if he uses his powers, will not do it to /full capacity/ all the time. Thus meaning, he will be using them VERY sparringly, or use many low-chance of success or low-effect damage powers frequently.
While that a wizard/sorcerer, even limited with spell slots, don’t need to waste more slots to make their spells more effective and follow their levels. An level 5 Wizard casting an Level 1 Magic Missile will be at Caster Level 5, no matter what.
A Level 5 Psionic Manifesting Energy Ray, without augmenting, will be using an 1d6 Ranged Touch Attack, costing 1 PP. To be equal to the Wizard, he’ll need to add 4 more PP(thus, making the Power effectivly an level 3 Power by itself) and boost it to 5d6 Damage.
Equal to most offensive spells Arcane Magic has at Spell Level 3.
Maybe not the /most/ balanced of it all(because frankly, ‘Balance’ and ‘D&D’ don’t exists in the same sentance completly), but I certainly find that it’s a good system for psionics.