Before we call the nerf hammer, lets think a little bit more about rengar
So apparently someone made a thread called "Why cant Kha'Zix use his W in air but Rengar can?", in hope for some buffs to Rengar. Firstly this is stupid because even though the cat and the bug are rivals, you cant really compare them like that. I could as well say, why cant Rengar get to jump everytime when he gets a kill, like kha zix does, so please dont compare these two. They both have their own problems, lets move on.
I agree he sucks to deal with, but seriously what else can he do than burst someone down? If he cant do that, he literally doesnt offer anything to his team. Ever since Rengars pre-rework he is not able to splitpush anymore, because his Q got changed. He cant teamfight very well either. And dont come here telling that he can burst someone down in teamfights, I mean sure he can, but hes gonna go down too. If he doesnt go down, then you maybe should check how fed he really is.
Another thing is the way how he works. Lets be a little bit more open-minded for a second, and think ourselves in a situation where we have a decently fed Annie on the other team. A teamfight is about the break out. Annie has her stun and Tibbers up. Now what is the first thing that comes to your mind? Obviously, NOT TO GROUP. Because if you do that, you are going to get destroyed. So you split and Annie bursts your ADC down, but meanwhile you have destroyed her team. This is just how Annie works.
Now how different is that from Rengar? If you split and go alone to farm to a sideline while the enemy has a decently fed Rengar. Maybe you should rethink about going there, because he is going to get you killed sooner or later if you do that. So group instead, thats his weakness.