Yuumi ruins ARURF
Unless healing AND shielding is reduced to 0%, she is broken because of the rate she spams it, with 0 counter-play since she cant be targeted. It is ARURF, which is meant to be a more fun environment. But Yuumi is not fun for at least 5 players in any game. She makes sure that one enemy champion will not die. EVER. No amount of ability spam will penetrate her perma-shield, and it it somehow does, she heals it back instantly. Best part is that she can heal through her "paired" champion's stasis.
Her drawback is supposed to be that only a single champion is in a lane which can be targeted. Meaning all attacks go to them. But when said target cant even die because of Yuumi's spam, this drawback is made redundant, and in face becomes a benefit to the rest of the team because 3+ players need to be devoted to just trade with yuumi and her "paired' champion. Her only other drawback is when she is by herself. But that will never happen with an immortal "paired" champion for her to ride on.