Something I saw a lot of Rengar players doing before his nerfs were just blindly leaping into the thick of fights and taking out the ADC while all 5 people were on him and he'd have enough damage to take out the ADC quickly while also having enough sustain to keep himself alive long enough to be able to stealth and walk away before the rest of the team killed him. Now, people still try to play him this way and I watch people try to leap into a group of 5 and immediately get killed. No champion should be able to do that--that doesn't mean that I think Rengar is balanced because I have also noticed a shockingly low amount of people picking him and he almost always loses his lane or jungle. I don't particularly like Rengar and have no desire to play him but I accept the fact that others do. Therefor, yeah, I'd like him to have some balance changes to him so people will stop feeding because he's too weak.
TLDR He should be able to assassinate the enemy ADC but he shouldn't be able to do so by himself with the ADC's team surrounding them, so balance changes should be considered but his kit makes that hard to do.