15 perks compared to 30? Thats a big difference. Plus you got like 20 other perks from completing the add-ons. So really its 25/50. Also those perks you get from completing challenges are useless. The challenge "Kill everything" only grants a 1% damage per level & there are 3 levels. 3% is waaaaaaaay tooo little to notice.
of course it's less, but sometimes less is more. fallout 3 took the quantities to such extremes, that you did not even have to think about what to choose, cause the game allowed you take almost everything anyway.
now you have to think and choose a bit, which i like.
and way is it too little? it's not like you couldn't dish out massive damage. i can one-hit kill deathclaws with my gobi, where's the point in an even higher damage?