Why Rengar is unbalanced

I have been a Rengar main for 4 years now and he's always been one of my favorite champions. I usually do pretty well on him and carry most of the games I play with him. However, the games where i don't do well just fell like complete hell. I mean and know he's an assassin, you get ahead, you win, you lose lane, you lose the game, but he just feels so one-sided. Especially since the q revert (which I love), adc's on the enemy team just dont play the game. How is killing a player in 0.01 seconds fair? But when you're behind, the fact that only the first person you see gets the bonus damage makes it so hard to something in a team fight since they can just put a tank there and you're done. Also, people who know how to play against him make him really unreliable for new Rengar players or even some experienced ones because he's relatively easy to counter.

If I could make some changes to his kit, I'd try to remove the "first person seen" damage buff on his ultimate and make it on anyone so he can be more flexible or just remove it completely but have invisibility instead or camouflage but for a shorter duration. I'd also buff the damage on his empowered W because its great to survive in a fight but when your in a 1v1 and have to use it, you just lost most of your damage so you'll probably die anyways. Finally, I know some people will scream and say he's already broken but honestly his wr is pretty bad and not a lot of people play him or try to pick him up anymore. I just feel like he needs to be more flexible instead of being a "one shot" or "don't do shit and die instantly" type of champion.

Give me your thoughts down below!