Yes, Psionic Body is underpowered – and so is Toughness
Both Toughness and Psionic Body are absolutely awful feats that should just about never be taken for their own sake. They do get a kinda-sorta exception though:
- The game will never get past level 1, maybe 2
- You’re playing a low-HD class
- You have really bad ability scores so your Constitution is low
- You have plenty of abilities already (i.e. spells or something similar)
Then it might be a ... not-entirely-awful choice. And Psionic Body can be considerably better in this case: if you are a psion, you start with a bonus psionic feat; that’s 4 HP. If you’re a human, you could take a third, for 6, twice what Toughness gets you.
But even in this circumstance, these feats are painfully weak. It is not a large amount of HP that you’re gaining in any of these scenarios. A feat for 6 HP is sort of substantial at 1st level but it won’t be at 2nd, and definitely not at 3rd. And the scaling on Toughness is just too slow; 1 HP/level is equivalent to a +2 bonus to Constitution, but only for HP. You can buy that.
However, all that said, don’t get the idea that HP isn’t important. It is. It’s just that these feats simply don’t improve your HP enough for their cost. Feats are extraordinarily valuable; you get very few, and some of them can do incredible things. You can, and should, get magic items to improve your Constitution. Spending feats to do poorly what magic items can do well is a bad choice every time.