Rengar is good in duels. Rengar will not survive if you guys are grouped up. That is what his whole kit is built off of. SINGLE target Q, SINGLE target ULT, SINGLE target E, he is built for solo kills.
If Rengar is able to ult and jump into your team, get a kill and walk out unharmed; that's when he is super fed. That's not broken at all, a Super fed Lissandra will 100-0 your ADC, a super fed Zed will 100-0 your AP carry without having to use the deathmark. A super fed ADC will solo carry with auto attacks.
I also guarantee that you have plenty of time to react to his ult if you have the skill to do so. Climbing the soloQ ladder tells people that you are a good player. In league of legends people are praised as good players if they have god-like mechanics. In reality, there are many attributes that makes a player good; and mechanics are just the tip of the iceberg, reaction timing is one of the many things about League that you need to have to be a good player. If you believe that YOU don't have the time to press 1 button to place down a Pink ward to see rengar, then you need to find a different hobby because League isn't for you. I myself have plenty of time to place a ward when I see rengar, because I've done it before.
This comment wasn't to call you out for being a "Silver scrub that needs to learn the game", this was to tell you that this is what the game is and is here to stay, if you don't like that, then leave our community and delete the game from your computer.