It's a mix of Riot being stupid and the game being impossible to balance 100% cause someone is always gonna be better or weaker than another at some point unless you want everyone to be the exact same.
Currently speaking there aren't really that many CHAMPIONS that are just flat not balanced. In fact old Poppy was the only one who was ever really THAT bad (at least for as long as i've played as I've heard release Xin/draven were nightmares) old poppy would be trash early game then EFFORTLESSLY run through an entire time under their power 1v5 and still come out with nearly a full hp bar. That's without being fed.
There is currently nobody in league that can do that without being fed. Rito on the other hand constantly ruins things with their item/stat balance issues and refusing to listen to their playerbase (which they do have some right due to most people only speaking out of salt instead of facts). As far as actual champion balance goes though there are a few cases of overloaded kits (riven yasuo camille etc.) who could use a few hits but all in all nobody is out of control