Sometimes if you nerf an ability directly, another characteristic of the champion will indirectly be nerfed.
It is the case with Rengar's sustain. His Base Hp/5 was nerfed (~ halved) a few patches ago to compensate the fact that W was too great of a sustain inlane. His Hp/5 sustain was aggressively nerfed because W needed a nerf.
But now, by lowering W's healing by 1/3, his sustain went down by alot since Rengars won't use Emporerd W as often and when they do it won't heal as much.
Since his sustain took a huge hit, it would be a good timing to put his Hp/5 on a more reasonable number. Right now it's the lowest of the game at any level, and by alot. I've always felt like if you have to nerf base stats SO MUCH because of the champion's kit, then do not look elsewhere: the kit itself is broken. Fix it. Don't kill a champion's base stats because of it.
A little statistic:
Rengar's HP/5 at lvl 18 : 11.2
Riven's hp/5 at lvl ONE: 11.3
Rengar has been a complicated situation.
Our initial approach to the champion was to chip at him until we put him in a good spot. Unfortunately, it turns out there are core mechanics on the kit (ie: % Health based heal, a huge Armor / MR buff, instant stealth) that simply don't fit the gameplay patterns we were trying to create for Rengar...thus we began to gut / remove them.
For example, his stealth duration was nerfed due to the fact that it was instant, but we have now made it non-instant. This leaves his ult feeling more lackluster than ever. The end result is a huge mistake on our part where Rengar is both weak and unsatisfying to play.
Classick and I are currently working on buffing Rengar back into a state where we feel more comfortable with his gameplay. Some of the changes include re-increasing the stealth duration back to create the prowling feel again and adjusting the W heal to be a flat heal based on champion level. We're still working out all of the specifics but we know we took a faulty approach to Rengar's balance for the past few patches.