Rengar doesn't feel like a hunter anymore.
No matter what his state of balance, his current playstyle barely resembles that of his old kit.
While he used to be this feared monster who feeds off of picking off strayed targets, now he's just an aggro magnet monkey-cat who's purpose is to jump in, absorb a ton of damage, heal it all back, then run away and re-engage in a few moments.
Most of the old Rengar community hates this new kit. Everyone who actually like the new kit are finding it fun for the wrong reasons. Yes, it's a lot more intuitive than the old design, but it doesn't deliver on the intended theme of being a hunter.
Going back to the old kit definitely isn't a good decision either, however. It had a lot of things that made it frustrating for Rengar's enemies to play against. The problem is that this new kit didn't make it any less frustrating to play against, but instead made Rengar's traditional playstyle just as infuriating to play in comparison to those on the receiving end.
Since Riot didn't spend too much time waiting to give Cassiopeia and Ryze another rework after their first failures, I really hope Rengar gets another one relatively quickly. I know there are other champions that need it more, but to completely the fun for 90% of Rengar's playerbase just feels completely unfair.