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.