Haven't thoroughly read through your guys' comments however...
From my experience Rengar is banned in at least half of my ranked games.
Feral Flare + Hydra + Youmuus and then tanky items is sufficient enough to take anyone but a tank out with ease.
Of course Rengar similar to some champions such as WW jungle etc have an issue with ganks pre-6, but it isn't the worst since you still have bushes and can use your empowered E to snare.
But yeah, I feel as though Rengar is often banned because of what he can do late game, easily kill a carry or pick someone off the enemy team. And because the deathtimers exceed 30-40 seconds you can take a lot of objectives or shove a lane since it's a 4v5.
Was just brushing past most of the text and I feel as though people have concerns in regards to HAVING a Rengar on their team yeah? If you can play Rengar well there's no issue with having him on your team. Rengars can single handedly turn a game around which is why I ban him often in ranked games.
Oh and in addition, you can use your empowered E on rengar right before you leap, but you must use your E then right click in quick succession or it'll pop you out of stealth and you can't leap iirc. It's a lot more efficient than leaping and then using your E since there's a high chance of you maybe missing or the enemy reacting with a flash etc.
On the other hand it could be detrimental to your team if you don't pick somebody off after using your ulti. if you're building glasscannon.
Usual build is Hydra Youmuus Last Whisper Black Cleaver Randuins and Mobos.
But you can forgo the Black Cleaver / Last Whispers / Youmuus and opt for more tanky items of course.
And if you're jungle, because of p reference, opt out one of the 3 damage items with a SotEL or FF.