League of Legends (LoL) Question: Status of Rengar

He doesn't assassinate well unless you build a considerable amount of damage. His sustain only comes from a 5 stack fervor charge. Sure he gets on the carry really well, but after that what? so many champions do his job better

His job is merely to hunt squishies. He has high enough base damage to do do just that even with defensive items. Even if he isn't doing damage he's pretty beefy with his W and has a decent heal to sustain himself or a root/slow to help his team lock down a target. Plus most Rengar players try to make team fights 4v5 before they happen by catching a carry alone in the jungle.

tell me what rengar does in team fights, tell me what he does mid-late game if he's not ahead. He literally does nothing. He doesnt have enough damage nor enough tankiness. About the only thing he does well is take towers

Nah best case scenario you get bruiser-y items like Bloodthirster, Blade of the Ruined King, or Black Cleaver and some defensive items like Randuin's Omen Frozen Mallet or Spirit Visage. His build needs to be kinda mixed.

I always saw Rengar as a full damage assassin but if I'm not fed I can't kill the adc and still help.

Thanks for the answers I will practice him a bit more

I don't think it's "He has to be ahead to be effective."
I think it's more like Nocturne, if you're ahead you can be a powerhouse with both damage and tankiness, but if you're behind you have to choose one.
Full Tank Rengar doesn't do damage and doesn't have any CC, so it's not great, and full damage Rengar basically instagibs a carry and dies right afterward, making fights a 4v4, but it doesn't even work well against smart teams.

Pretty much every champion is scary when fed. The problem is he has to be ahead to be effective. He has extremely strong levels 1-4, but if you can't manage to get a significant lead you will fall off. His damage is pretty poor late game, his tankiness is okay, he doesn't really do anything in team fighrs, his split push is pretty good but other champions do it better. Honestly I think he needs a rework

As for build, if you are snowballing, his necklace is still good, along with bloodthirster and Blade of the ruined king. If you're even or behind you can go with a more tanky/utilty item like Randuin's Omen or Frozen Mallet. Personally I build Sunfire Aegis and Spirit Visage for defenses.