Lets talk Yuumi buffs
Yuumi is so underpowered. Every healer does what she does- better. More damage, more heals. All yuumi does right now is troll the ally adc.
HIGHLY recomend Slow based on distance traveled on her Q. Have it stun at the last 20% of her NON GUIDED Q.
W needs a buff, not enough damage to make it worth leveling.
Her E is her only balanced skill on release.
Her R both doesnt do enough damage nor does it root fast enough or long enough to be viable.
Lastly, Yuumi needs either more raw AD or more tanky stats.
Shes weaker than a Janna and with less peel.
Shes weaker than a Rakka with less heals.
Shes weaker than a Nami, Less CC.
:( Please make this champion good! Great idea for a supp just huge failure to deliver.

Please note: The stun on her Q is at the END of her max range only. She would NOT have stun available when on ally shoulder- It would become the range-based slow.
Nami has 2 hard CCs and can shoot over minion line (similar to how Yuumi Q can go around minion line) AND, a root doesnt prevent the enemy from firing back like Namis CC does.
By making her Q have a stun when unattatched, it gives her disengage ability to some minor extent which all enchanters need. This play style also forces Yuumi to jump and bounce around- alot. Itll keep her fun, fresh and with a high skill cap AND be totally viable champ.