Now this is my opinion and philosophy on video games. It is most likely not true, but this is how I see it. Someone who is unfun to play against forces you to learn their strengths and weaknesses and try to not play by their rules.
A prime example is illoui. If you don’t stay in motion, keep dodging skill shots, don’t watch her mana, and don’t build armor, she will make sure you tear your hair out. You do all of that, illoui will have a bad time. This should also encourage teammates to help you. Like “oh crap, he’s having trouble against yasuo. He’s a late game terror so we’d better help him out for all our sake”. Same goes for other games. Nobody likes snipers in cod, halo, or counter strike, but if you stay out of the way and force them into cqb, they’ll struggle.
Whether annoying characters/weapons/builds are balanced is debatabe, but at the same time if they are that annoying they are doing their jobs right. Plus it’s a guilty pleasure for a lot of people, a good stress relief to some, and there are masochists who like these games.
Like I said, I might be wrong on this, but this is why I think there needs to be annoyances in games. It makes you think more about strategy, and learning the opponent.