The largest, and some would say ONLY downside, to play Yasuo is that you have to live with yourself afterwards. Playing Yasuo erodes the soul. It's like taking a cheese grater to your humanity and sense of accomplishment and going to town until eventually nothing is left but a dark, empty feeling where once your self-worth existed. From there comes a strong level of defensiveness. Anyone besmirching you is just "mad" and anyone speaking ill of the champion you have sacrificed your soul to is just a hater or a rager who can't see how great he is. Once things have hit rock bottom, you will preemptively "defend" yourself from what you're certain are attacks on your character... tossing insults at others in-game or gloating over kills and victories before the enemy has said anything aggressive or insulting to you at all. After all... you KNOW they're thinking it.
Paranoia sets in and you feel compelled to defend yourself from verbal attacks that you're SURE are coming... becoming crass, rude, toxic and arrogant to other players to "show" them for those insults they were assuredly thinking of when they saw that you main Yasuo. They were JUDGING you! At this point, you cannot abandon Yasuo to play other champions... as you've sacrificed so much of yourself to him already. You cannot give up the power you have after already paying the price for it. So you play Yasuo again and again, each game more defensive and unhappy than the last. You thrill in your victories, but the feeling is hollow... You tell yourself you have earned it. That it is skill and outplay, and that you are a rare breed of hidden pro... and that if only the scrub teammates you get every game weren't so bad you could easily carry yourself to silver. Losses are dismissed... pushed away by the wall of arrogance and the illusion of mastery and skill you have built around yourself... and thus, nothing is learned. Nothing grows or changes. Every game comes and goes like the wind... momentarily ruffling things but changing nothing about your ranking or yourself... forever. Your tragic existence simply shambles on... to afraid to accept the mercy that would be ending it.
TL : DR : Yasuo has basically the same downsides as 
