The fact that he has an infinite scaling shield, infinite dashes through any enemy, as well as his wind wall that has no damage limit and can block even after projectiles have made it's way past the wall only to be blocked mid animation, and his 100% crit at low levels. It makes him incredibly broken at bronze through gold level of play. I won't say plat or higher because I know I will be flamed and challenged with "well the pro's don't use him for a reason!" Go fuck yourselves for whoever says that. Balance in the game isn't for the .5% of people who are diamond or higher. (Not saying it isn't) It's more meant for the causal players and lower tier players to give them a fighting chance to come up in ranks.
The fact remains at low level of play Yasou is toxic. His kit has way too much going into it for his offensive capabilities. I know people are going to say that he is a glass cannon and all you need is cc. But take into account (not ranked) but normal matches [blind and team builder] which case you don't have most of the CC you would want to 100-0 the guy. Not to mention that even if you do have the CC to stop him the coordination you need to take him down is just beyond the fun of the game.
Bottom line, you should always be aware of champs and have to adjust play and champ picks to counter against others. However, you shouldn't have to adjust whole gameplay and ALL selections just to counter one person. It makes the game no fun and just makes everything so toxic for everyone (but the people who main him)