Couldn't a bronze 5 Yuumi theoretically attach itself to a diamond smurf ADC (or any other champ, later in the game), and duo queue to.... Close to diamond? Literally carried to diamond as a literal backpack? Because if Yuumi doesn't need to switch teammates because one teammate is way more worth attaching to (skill-wise) than the rest... Then she's untargetable for the whole game basically, right? Am I misunderstanding something?
Yuumi needs to hop out of her ally in order to autoattack and activate her passive, getting mana return to keep casting her expensive abilities and creating the shield she uses to protect allies. This creates a widow where enemies can punish her, however brief. Knowing when it's safe to come out and fight will be a big part of what separates the good Yuumi's from the bad ones. If you just sit in your ally the whole game, your kill pressure in the early game is basically nonexistent. Even Soraka can put out good damage and zone the enemy away, but Yuumi's cooldowns are quite long early on. She needs to be out and about using autos in order to have presence in lane.
She also depends on the positioning of teammates to keep enemies in range for her Q and her ult. On an immobile ADC, she'll struggle to get that CC and damage out where it counts, so she needs to hop to the frontline tanks or even assassins to make sure her CC is hitting something other than the enemy frontline.
If her attached ally dies, she doesn't get a free recast of her W to attach to somebody else, so if you don't position yourself on the right teammates as the fight comes to a close, you can easily get stuck out in the open with nowhere to run.
Still, the premise behind Yuumi is that she has a lower skill floor than a lot of other champs. She's designed to be accessible, but there's nuances to managing her resources and attaching to the right people, so her skill ceiling at least exists.
There's way more skill expression than simply moving.
Her skill expression is in playing keep away from the enemy. She's made of wet tissue paper, so jumping out at the wrong time can get her annihilated.
She can still take damage while attached..... In other words, she needs to detach to heal herself.
You can kill her while she's switching from ally to ally. Also, her AA range is pretty short (around 450-500ish iirc) so it's easy to harass her when she's going in for the shield. Her Q is a very slow projectile, and even though you can control the flight, it's still relatively easy to avoid.
If you're just attaching to a marksman you're a good Yuumi, not a great Yuumi.
