Singed is very unhealthy
Singed isn't the funnest champion to play against, but for most of LoL's lifespan I'd argue the pros Singed offers to the game outweighs the cons. His kit and play style are interesting and unique, and have their moments where they're fun to play with and against.
yet riot nerfs champs like rengar because he is unhealthy when rengar clearly has weakness.
Rengar doesn't have many pros going for him other than his cool aesthetic and his rivalry with Kha'Zix.
Otherwise his kit is just really obnoxious to play against.
His Shtick currently is also essentially suiciding into an enemy team and deleting a squishy, which he can do pretty reliably with little outside of "hope he's 10% short from a 100%-->0% OHKO" often being what it comes down to. Sure putting him behind and counter-building are ways to play around him, but that just comes back to "hope he's misjudging his burst".