The biggest problem with Rengar is that when you compare him to the likes of Zed and Leblanc, there really isn't much room for him to go wrong with him offloading his damage.
Say what you will about Zed and Leblanc, they at least have some ways of messing up their abilities. But for Rengar, he can just ult and right click, and then press Q the moment he pounces on someone. Sure, you could argue that Rengar's weakness is that he could pounce at completely the wrong moment, wrong champion, etc. But that comes the territory with assassins. They're high risk characters that need to assess a risk before going in.
So why pick an assassin like Zed and Leblanc, when Rengar can offload the same amount of damage with such minimalist effort?