As the title implies, her design is very useful when playing with something like a Renekton/Jayce or basically anything strong that can dive in like that, you can poke safely and not risk getting punished and you can help your diver pretty hard. But with marksmen they just need to keep their distance so there's not really much to do.
In fairness, adc's don't really know what to do most of the time when they're "alone" and being attached to seems to put them into the flight or flight mode where they go when you roam as a support and they just either get mad and play hella safe and flame you or they panic and start trying to 1v2.
It's not even Marksmen. It's with people who respect her, work with her ranges, and actually have over 100 games playing (or at least the skill to feel like it, since many horrible players have thousands of games).
Tonight...... I've literally had shit adc after shit adc after afk jungler after afk jungler.
It's like they don't even want to PLAY. I'm outdamaging my adcs on YUUMI. They don't poke ..... ONCE in lane. Finish game with 8000 Champ damage. WTF.
Or I get the Hecarim whose literally after jungling in between every single ult....., and doesn't even gank a lane prior to 14 minutes.....
Or a Zed who literally int feeds the entire game.
Xin Zhao that 1v4's and doesn't get a single kill out of it.
2 Ezreals who do 7-8k damage in 30 minute games.
Lucian who literally doesn't exist because he's too scared to exist vs a MF/Lux (WTF...... Lucian stomps MF.....)
I disagree. The speed from the heal is extremely good for getting your ally into range to finish a kill, start a trade, or get out of a bad one. Marksman need to keep their distance, but that blue ring is huge. If they can't stand that close comfortably (assuming there isn't an artillery mage or something), then they're a pansy. The speed + slow is great for spacing early on, and you give your marksman a ton of AD later on, and they can probably make the best use of all that AD.
Had good games as 
.
Yuumi to me as an ADC main feels like a Support you should play IF YOU ARE DUO WITH THE ADC
I don't think she'd work too well if you and the ADC aren't on the same page.
its sad people think this about her she has so many builds its ridiculous lol.
She power spikes your power spike. Sometimes its the adc, but you'll probably get more bang for your buck riding a fed/competent top lane champion or jungler. I had issues with her upon initial reaction, still think her invulnerability mechanic is problematic due to allowing her to completely opt out of laning phase risk, but I developed an appreciation for her kit and what the designer was trying to do. I think the person who made Yuumi really thought about how she would impact gameplay at all stages of the game and did a good job. I also still feel that once people get a better understanding of how she can be effective she will mess with the balance of champions she's effective when paired with. If Yuumi is going to stay, the game will need more options like her. I think certain abilities should be able to knock Yuumi off her anchor, there should be some options to check her permanent invulnerability, its definitely an abusable mechanic.
Had a game with a Nidalee Yuumi combo. They never died (double heal) and caught out everything (Spear and slow). I basically got carried 2v5
