First of all, a lot of her early game poke is being nerfed.
Q has increased mana cost, less AP ratio scaling, and the flat increase just doesn't feel enough to compensate at early levels.
The W gives you +7 AP +4% AP at max rank, but it now has a channel time, can be interrupted, and you hop off when an ally is teleporting...
So let me get this straight: You can no longer TP straight into lane, and you have to walk/hop on your ally?
AND if you get silenced, you get fucked over?
I'm fine with the 5s CD if you get hit by immobilizing CC, but we're in such a damage heavy meta, Yuumi is gonna get fucked if she detaches late game, especially in lower elo where people don't know how to position properly with her. In addition to this, the changes are basically forcing her to hop off more, which wouldn't be an issue, except they're not giving her any compensation for her higher risk gameplay. She now has very high mana cost, and she must sustain herself by risking her life to do so. This makes lanes such as Leona and Nautilus absolute cancer to play against, especially combined with how risky her W is now.
The E charge removal is also a nerf.
Now you can't store two rounds of movement speed ups, and they also cost an ADDITIONAL 70 MANA to use at just rank 1... for an additional 40 health.
They also knocked off 10% movement speed, but gave it an attack speed buff, which is nice, but I pretty much accomplish the same thing with Ardent Censer anyways, which most Yuumi players build fairly early on. Unless the AS buff stacks, it's pretty pointless, as Ardent Censer was such a great cost-efficient and good synergy item on Yuumi.
Also, while one might argue we have stronger heals now, we had pretty strong heals before because every single sane Yuumi player has an Athene's and a forbidden idol item, which could heal half of your ally's HP bar anyways with a single Q charge for your Athene's. The bonus scaling and flat scaling just get more irrelevant the later the game drags on.
While I get that these changes are supposed to balance her for solo queue while keeping her fair in pro play, most of us aren't pro players.
I'm perfectly happy seeing Yuumi permabanned in every single World's game if it means the other 99% of the playerbase can play her.
Some parts of her kit are unhealthy, such as her untargetable mechanic, but it doesn't make her broken; people are so used to being able to one-shot squishies that the moment they can't, it's considered OP, even though assassins can shred through her target in no time, as long as people buy GRIEVOUS WOUNDS. Seriously, as a Yuumi main my delight is when idiots don't buy any form of healing reduction then complain about how I can keep my ally alive.
That's like complaining about an ADC doing too much damage and not buying any armor.
Also, her winrate is at 40%. That is not the winrate of something broken. Frustrating to lane against? Sure. But her winrate shows she doesn't have as much agency over games as people like to pretend she does.
At this point, I'd like to propose we keep Yuumi as she is, or make her have some Kalista-like feature where she can only hop on to certain players, to make it more difficult for pros to synergize Yuumi with every single person on the team while still keeping her viable for solo/duo where you mostly stay on your duo/the carry. I also saw someone on boards propose that she gets "drowzy" if she stays on an ally for too long, which reduces her damage, but it resets if you detach and reattach, which is a small nerf that would make her a bit more targetable, if a 40% winrate champion needed a nerf at all.