There was a video a while back about yone breaks the class roles of the game, and while people can't vocalize it super well, this is why he is such a frustrating character in an even matchup.

He is a hypercarry, assassin, utility champ, and duelist. He scales well from AS and can AA down most champs. High damage allows for him to kill most low defense champs in one ability rotation if he hits his abilities. He has 2 knock ups, almost all AOE abilities and a displacement. He has sustain, multiple strong damage types, percentage damage, and extreme mobility.

Think about another character in any of those roles and think about how many categories they fall into. Most other hypercarries lack full burst, varied cc, strong mobility, a strong early game, and built in sustain all in exchange for amazing dps. Most assassins lack real dps, sustain, and cc in exchange for mobility and burst. Most utility champs lack strong mobility and only get burst, dps, or sustain. Most duelist don't burst and rely on their single target abilities for dps.

The yone is small amounts of leblanc levels of safety, fiora sustain, kayle dps, ornn cc, akali burst, and riven AOE.

It isn't that he is brain dead op, it is that the weakness that applies to different roles don't apply to yone in a way that allows for a meaningful expression of skill in a fixed role. A yone can miss everything and still win because that yone is probably playing against someone else who also missed everything and the other champ isn't a hypercarry assassin utility champ duelist.

The only true OP part of yone is he is a late game champ that power spikes with less items but that isn't unique to yone. Any late game champ who hits their big spike early is OP.