1. Yuumi cannot 1v1.
She needs a quality of life improvement on her Q and E, and forced off her host every few seconds.
What other champ cannot 1v1 in this game at all? Maybe really tanky champs like blitz crank? But he is a tank and peels for the ADC, so he can take it. Yuumi is easily 1 shoted. Maybe take away some of her squishyness?
2. Yuumi is making ADC's feel like they are 1v2
My solution to this problem and to make Yuumi users auto attack enemy's is to have her kicked out of her host's body every 6 secs or every so often. This way you have to auto attack and you can pop back in for a heal and a skill shot.
Right now you can stay in your host's forever, or until they die. This is not fair to the ADC who is constantly being targeted. This is the most troll design you could ever put on a champ. Talk about tilting your teammates. She has to be forced out periodically so the ADC feels like they are not by themselves and also, more afk prevention this way.
Edit: Blitz can 1v1 at his tower.....