Yuumi's might be gutted after release like Irelia was, but for different reasons.

We've seen Rakan. We've seen Irelia. Now let's welcome Yuumi, the newest introduction into League of Mobility! This actually isn't too much of a problem for Yuumi as long as her AP ratios are fine.

From what I can tell, she already looks like a high skillcap champion. You'll likely be putting in a LOT of effort to get a very small amount of benefit. Her shields shouldn't be overbearing, her skillshot is hard to aim, and her heals should be relatively small. At least, this is how it should be. If her AP ratios on her heals and shields are similar to other enchantress', then I see problems with how she's designed. Being able to shield and heal the same amount as an enchantress that has to risk their neck to provide those ratios makes for unfair gameplay. Knowing Riot, if they ship her with numbers too high, they'll handicap Yuumi and pull her numbers down to unimpactful levels.

By making a support enchantress hyper mobile, you've ensured that she'll be powerful in the right hands, thanks to her ability to avoid death until every single other champion on your team is also dead. Her ultimate also has massive potential. In return, she faces a hard to aim skillshot as her primary source of damage. I'm looking at her as a form of Sona that doesn't provide team wide benefits, but does receive invincibility. This is Sona 2.0, and I'll explain why.

However, just because all of these things apply doesn't mean she can't be a balanced champion. It just means that she will face several issues. First, the AP ratios on the Shield and Healing should be balanced according to their cooldown, to the point where it's much easier to heal and shield as other enchantresses. Second, she'll be facing

Currently, Yuumi seems to have a lot in common with Sona. Similar ultimates, heals and shields, and some poke damage. The main differences between them are Yuumi's mobility contrasted with Sona's team wide support, and instant stun. While Sona seems like she'd impact teamfights more, I foresee Yuumi being great in skirmishes with her ability to instantly become untargetable, perhaps even at low health. Both provide in Lane, both provide CC, and both provide survivability to the rest of the team. The only question then is if your team can provide support for you, so that you can provide support for them.

Of course, Yuumi's utility is handicapped by her single target survivability. That being said, being unkillable for the entire duration of a fight seems a bit too much to me, so her numbers are either going to feel really low, or she's going to get other aspects of her kit taken out. I'd like for Riot to release her fully functional and balanced, and I'd like for her not to be overbuffed in a hotfix 2 days after release. She looks like a champ with a high skill cap. Give it some time, Riot.