Rengar is overpowered as fuck but he's a hundred million times better than the old ones
His numbers are the reason why he's so strong, not the actual kit. His upfront burst is composed of crit auto+Q and the Q's damage is avoidable and the frontline can make it a lot harder for rengar to reach the adc because camouflaged champions are revealed when they get too close to something. The hard cc cleanse is a tradeoff of damage in order to continue pounding someone and that is not "unhealthy". The CC immunity is.
Once rengar's numbers are in line, he will be a lot less bullshit-ish than the old rengars.