Originally Posted by
Duilliath
The AD/level isn't the big deal. It's the added abilities he gets. The extra range on his jump can make a huge difference, for example.
I don't see them do well because, much like Shaco, they rarely get to be in the position where they get to do well - ahead. Rengar has major problems once he falls behind. He can't easily overcome that disadvantage. His unique item will not help him here, same as all snowball items. If you are 0/2/1 your snowball item will do nothing do you.
I personally have not seen him being able to snowball hard enough to completely shut down a game, but I'm open to the possibility that this might've been specific to the player rather than the champion. I have seen the odd Rengar do well, but that never once was when he fell behind.
By comparison: leave Jax or Olaf alone for awhile and they will still obliterate you. Rengar can't get enough power once behind.