The largest problem with Yuumi now is that she has poor synergy with ADCs.
"Yuumi and her Anchor gain bonus attack damage or ability power (Adaptive) equal to a percentage of the other's highest stat. Additionally, Yuumi starts with a point in You and Me! and Prowling Projectile Prowling Projectile instead has 6 ranks."
So, if Yuumi attaches herself to a ADC, she gains AD not AP. And of course she has no AD scaling nor anything to do with it. So if you really wanna play Yuumi, play a mage botlane duo. Especially Veigar which you can amplify and siphon 21% of his ludicrous AP and make him buy a rabadons which will boost his AP an additional 40%.
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, etc pretty sticky.