I find rengar pretty shitty and is a heavy 'Feast or famine' play style.
He either can One shot anything he wants. Or just... die I guess.
he isnt at all useful when behind. But when hes ahead. Hes INCREDIBLY frustrating to play against, specially if you dont Have Ga, Stopwatch, Hourglass, A support who can peel easily, Have the jungle warded and have eyes on him continuously, Dont have any Escapes, or any CC.
So, If you let the assassin do his job, Yeh hes pretty shitty to play against, because well. Hes doing what he is supposed to. Kill an greedy squishy players.
(if you dont build against a fed rengar, specially you KNOW he is in the game and getting fed, and you're dying before you can get an Auto OFF... You might want to not build that 3rd infinity edge as jhin when your support is a Veigar.)
(If you go try to take red, or try warding Baron or drake when there inst another vision in your jungle.)
If that all else fails, Just try to one shot him first before he does.
Overall, Rengar does need the help. However. I Really hope what they do to him isnt just "Here have more damage" Because that doesn't help anyone.
Maybe skim the damage from his kit for some defense, or clear speeds, or CDs.... maybe apply q On turrets/Inhibs.(I do not know if this is current)