*deep breath*
Rengar, when winning, is extremely hard to deal with. He has free sustain, tons of damage and ton of survivability. I speak as a rengar top player that he's incredibly snowbally, much like classic riven. I say "when he gets the first kill" because he has the toolkit early game to do just that easily.
I find him viable top and I have a lot of fun playing him in that role. I win most of games when my jungler gets me the first kill. I lose games when I get bullied out.
Could he use some love? Perhaps. However, as a Rengar player, I find him fun, ambushy and super risky. Adding too much baseline power only would make him incredibly frustrating.