As someone who bans Rengar, I wish I didn't, but here's why:
1- If I'm playing support, Rengar's immune to my CC, I can't burst him down for my carry unless I pick the appropriate support, and if I'm not last pick in draft, I won't take that risk.
2- If I'm playing adc, the second Rengar hits level 6 we're playing cat and mouse, and I'm the mouse. League turns into a completely different game.
3- Rengar has killed me behind 4 lotus traps and a control ward, under tower, in my own base. Inner turrets were -not- down. Yes, I literally made a fortress I could run to when he Ulted. Yes, he followed me to it and killed me in it
4- One way to counterplay Rengar involves teamwork. Teamwork in soloqueue? 'Nough said.
5. Another counterplay to Rengar involves wards. Warding in bronze > silver? 'Nough said.
So he's kind of OP just in the way that he exploits poor playstyles so... effectively.
Assasins don't seem able to carry unless the enemy team is baited, makes mistakes, or is a pick/kite comp (which happens a lot- soloQ). Also I think it's just the way he kills- you have your APC and ADC shooting an entire team, they both killed you. But the assassin has to focus on one target to be effective, so there's this very clear 'Rengar killed you' moment in people's heads. There's also the insanely quick burst window, which I've dodged a grand total of once (with flash)- but since my team didn't care, 'adc newb!' it just postponed death for 3 seconds.
).
