So you don't want Rengar to be able to oneshot anymore...well..
Why? Because that's literally all he can do now. I agree he's frustrating to deal with but...
He can't splitpush anymore like he could before he got reworked. His teamfighting is miserable. All he has left is his ability to kill someone.
He is an Assasin, just like Talon and Zed. Assasins are supposed to burst down the squishy, and get out. Even though he really doesn't have any escape abilities like Zed does, when he has jumped in, he's in.
Also, watch his build. If it looks something like this
I'm pretty sure many champions could burst an ADC down with no defensive items. Talon for example.
When you face an Annie with her stun and ult up, you don't want to group right? Doesn't in work in the same way for Rengar, but the opposite. You don't want to walk alone, or he will come and hunt you down.
which costs an item slot to whoever is carrying it. The exclamation mark isn't a "fair warning" at all due to
. buff eve 2016