How many assassin champions can 100-0 from stealth a fed carry after getting dumpstered in lane with half as much gold worth of items?
This Rengar build basically makes it so you hit 6 and then you go bot or mid for your free snowball.
Take away the stealth for a second.. How many assassins can one shot a squishy? Quite a few. With few items? Again quite a few. The twitch, leblanc or lux didn't build a lick of defense until the Rengar was fed beyond it mattering.
As far as the stealth, isn't that the pot calling the kettle black? Twitch getting owned by stealth Rengar? Plus there's plenty of visual and auditory cues that there is a stealthed Rengar in range unlike Twitch or Eve.
He is still flawed in so many ways. He'll one shot someone then stand there looking stupid while you own him with that build. He has no more escapes or defenses until he regens the ferocity. Ryan just happens to be an excellent Rengar and can make the most of it. His team happened to do very well that game as well. Still, the rengar had the second most deaths on his team, even with the number of kills he had.
All that said, I do think stealth by itself is a rough mechanic in this game. It's never really felt good. Evelynn is probably the best implementation of it but still isn't great thanks to always visable pink wards.
There should be 3 minute invisible wards that ONLY detect stealthed things - Rengar, Eve, teemo shrooms, wards, etc but provide no normal vision that costs the same as a green ward.