perfect balance allows all champions to theoretically have 50% winrate if played by gods.
the biggest problem tho isnt balance but design flaws.
a good example is azir. he has super low winrate despite being the strongest midlaner in the game. how can that be? its simple, his power is much harder to use than other champions power because of how not a retard you have to be to play him. yasuo players and fizz players will always feed and suck with azir.
tho this doesnt mean azir is badly designed. its actually the opposite. even though azirs kit has some flaws, his power-utilization is wonderfully designed and every champion should be like that. if you suck with a champion, he should be super weak and if you are really good with him, he should be super strong.
the more mistakes a champion forgives and the stronger his physical condition/abilities are by default, the easier it is to play him and the more successfull that champion will be statistically.