Pretty much as the title claims: Yuumi in her present form is far to strong in ARURF, and is incredibly un-fun to play against. When paired with champions who can take advantage of her adaptive scaling, or the additional mobility and healing she grants, there's almost no counterplay available. Champions such as Hecarim, Rengar, Fiora, Jax, Vladimir, Jhin, fizz, the list really can go on.
Normally I just chalk up losing games to being unfortunate. The Yuumi games, however, are absolutely terrible to participate in, and will often end my night of playing League. Building a Thornmail and Randuins on Darius isn't enough to stop the Rengar + Yuumi from killing him with a single right-click, let alone having any chance of escape. If you were to flash away, the speed boost from Zoomies makes it only a stall tactic at best. If another ally were present to make it a 2v2, the amount of damage taken from the initial engage is too high to even have a hope of turning the fight. Similar situations arise with Hecarim plowing through any remotely squishy champion, then sprinting away leaving no chance to retaliate.
The lack of enjoyable gameplay also applies (though to a much lesser extent) to allied Yuumi games. Too often games are decided solely by what champion Yuumi decides to attach to, rather than any effort made by either team. Many games are spent idly taking uncontested objectives, while whomever Yuumi is attached to rampages through the enemy team. It may be a spectacle to behold, but I think it lost its novelty during the previous release of URF.
I don't think yuumi should be removed from URF, but heavy nerfs to her W's adaptive scaling at the very least is called for in my opinion.