Yummi isn't bad, but merely suffers from unfamiliarity

Lets clarify:

When Bard was released, everyone was horrible with him. Everyone kept trying to play him like a spellcaster mage, and as such, fed their asses off with him. He needed minor buffs, but nothing too big to the scale everyone said he needed. When riot decided to buff him, players actually realized how to utilize him, then he was really strong.

When Jhin was released, everyone kept trying to play him like lucian. They whined about his reload mechanic, his low attack speed, yada yada yada. In short, they were trying to play a sniper character like Fox from Super Smash Bros, and that kept killing jhin players, and since jhin doesn't have any escapes, they died a lot. They whined, and Riot buffed him, and then people were like "Wait. What if.... instead of feeding.... I play back?" Then he was amazing, and riot gave jhin a buncha nerfs.

When Taliyah was released, players kept trying to play him similarly to viktor or syndra: a battlemage. They kept dying, players kept whining, then after buffs they were like "oh wait. Taliyah isn't a battlemage: shes a zone mage." Then she was a balance problem because low elo still kept trying to play her as a battle mage while middle to high elo realized how amazing she was.

When Aureleon Sol Was released, people just complained that "I can't do anything against assassins!". Then someone played him in the LCS, and people were like "Ohhhhhh, I have to Roam and be backline instead of frontline".

Same song and dance with Yummi. Players are trying to play her like rakan, or anyone with a flashy engage. Yummi isn't about flashy plays, but rather mediocre plays that massively benefit those who know how to execute them. She's incredible, and a stupid strong teamfight presence. Give her time. She needs no buffs, players need to learn how to play her, and she will be guchi.