Yuumi Is Not Weak, This Is What Yuumi Newbies Are Doing Wrong
I've noticed it's a pretty widely accepted opinion that Yuumi is weak, but she absolutely is not. Even her laning isn't terribly weak once you get the hang of it.
Here are some really quick tips for new Yuumi players. Please note that these are GENERAL guidelines and some matches or situations will vary. This is pretty basic but it should help those who are super new to her.
Champion Select - Yuumi works best with mobile teams and ADCs. Yuumi is not a good first pick or one trick. When playing her in ranked, please use her primarily as a pocket pick. Yuumi's kit allows her to skip on flash and opens up a slot for a summoner spell that's better in fights. You MUST skip out on flash if you're wanting to go even in 2v2 fights early on. In the future you can tell your ADC to go TP or barrier while you take heal+exhaust/ignite, but for now while Yuumi is new, don't ask your ADC to switch the summoner spells that they're comfortable with: just go exhaust+ignite.
Passive - Your passive shield is what will allow your ADC to go even in lane. It is legitimately the ONLY way for Yuumi's ADC to go even. If you don't make use of it, you will lose HARD. ANY time your shield has been broken and your passive is off cooldown, your NUMBER ONE priority is proccing your passive and getting that shield on your ADC. To do this safely, only attempt to proc your passive while you have your W up so you can instantly go back to your ADC. Against a champion with a hard CC that will kill you if you get hit by it (Morg Q, Lux Q, Blitz Q, etc.) pop back to your ADC with W the INSTANT you see them casting anything if you're not confident you can dodge/there are no minions in front of you. It's worth missing out on your passive for the next couple seconds while your W recharges for them to waste their mana and CDs.
Q - Only Q while attached. Literally do not cast this unless you're attached. Q every time it's off cooldown, but make sure you ALWAYS keep enough mana for another Q and 2 charges of your E in the event of a gank. Out of lane, pretty much just always have a Q roaming around in unwarded bushes as a scouting tool. There's really no reason not to just always have one out when it's off CD. Out of lane, spam your Q anytime you see an enemy in order to stack your Athene's.
W - Not much to say about this ability honestly. I'll just list times when you want to detach. Assume if none of these conditions apply, you should be attached. -You're helping your ADC push. Please detach and help your ADC push with autos when they're trying to shove. -Your ADC is taking a turret. Help your ADC take towers and get spellthief procs. -You need to proc your passive. -Your ally is going the wrong way. This may seem obvious, but sometimes it's easy to forget you can move on your own later on in the game. If you see a fight you need to be at and the player you are attached to is not walking there, please detach and walk to the fight yourself. Same for if you need to ward an area and your ally isnt walking close enough to it. Just detach for a second and place the ward instead of spam pinging your ally to go there. One last thing to note is that you should only detach when your W is off cooldown. It's your only escape and it's too dangerous in and out of lane to detach without your W immediately ready. This is especially true when you're trying to proc your passive.
E - In lane, only use this if your ADC is below 50% health. I've seen Yuumis use their heals to top off their ADC in lane, and that's a massive waste of mana. Never use your heal on yourself unless you're below 30% health or need a speed boost. You shouldn't even be getting down below 50% health in the first place. Out of lane, use this anytime you have 2 charges in order to speed your ally up so they can get where they're going faster. It's a waste to leave it capped at 2 charges unless you know a fight is imminent.
R - Late game this is a low CD and should be used generously. Whether it's to peel for your ADC or to help engage . Please keep in mind you can E and W while this is active. If one player moves you out of position while ulting, W to your next best target.
Max order - ~~If you build coin, max W. If you build spellthief's max Q. ~~ Max Q at all times. ~~Whatever you decide to max, generally I'd say it's best to just alternate points in the remaining two abilities once you've finished maxing your primary ability. ~~ Max E second. Alternating points in the remaining two abilities isn't terrible, but the CDR you get from leveling E is too important.
Rune Setup 1 - Aery, manaflow, absolute focus (!), gathering storm Magical footwear, future's market
This is my primary enchanter build. Absolute focus is great on Yuumi because you pretty much never lose HP and therefore increases the stats your W gives allies. I take scorch into lanes I can poke out, and gathering storm into lanes that I'm planning on going even in. Magical footwear is good because Yuumi doesn't need early boots like other supports do, and future's market is great because Yuumi is a relatively underwhelming support until she buys Athene's, so you need that ASAP. On the flipside, if you dont want future's market you can go the 5% CDR rune which is good because you have two very effective summoner spells and getting them up faster is pretty sweet.
Rune Setup 2 - Guardian, font of life, second wind/bone plating, revitalize. Absolute focus, gathering storm.
Pretty self explanatory. I havent personally tried it but it's supposedly pretty good into certain comps.
Core Items: -Spellthief's into low sustain, coin into high sustain. -Athene's. Rush this item every match. It's pretty much the only item I'd say is absolutely core. I usually get between 5k-8k healing with Athene's alone. It's actually insane how good this item is on Yuumi. With the 2 charges on her E, you can almost instantly get two fully stacked Athene's heals off just your ult if you space the heals out about a second.
That's really the only core item. Mikael's is also really good vs heavy CC comps. Due to the fact that you can't run away on your own, it gives both the ally you're attached to AND you an escape. Zeke's may be good but I havent personally tested it. I feel like any item with defensive stats are sort of wasted on Yuumi because you will be invulnerable most of the game anyway.
Oh yeah, and don't get upgraded boots please.
Thanks.
first, then I like to finish
and buy
. This is a point where I disagree with OP -- not having move speed when you need to cross some space yourself to move to a different target is bad, as later in the game you want to spend as little time as possible exposed. After that I currently go
then
. The HP lets you take one or two non-cc hits while if your buddy dies, but mostly is wasted. However, the empowered heals and shields speak for themselves, and the extra slow on your q is huge for catching people out/cleaning up stragglers. After that I finish with
, as you pretty much have all the stats you need, may as well make your chip damage a little more meaningful. I'm still experimenting with items, so I don't know how optimal this is but it has worked for me.
could be an interesting final item, giving her much safer and more reliable procs. in addition, because of her super low max HP, she can actually gain insane value from precision as a secondary rune choice.
AERY>MANAFLOW>TRANSCENDENCE>GATHERING STORM
PRESENCE OF MIND>CUT DOWN
this setup gives some insane ultimate cd, and cut down always grants her bonus damage since she has the lowest hp in the game.
my final build looks like this
Cooldown boots for what exactly? You don't need cdr at all on yuumi. Get Presence of mind in precision. I mean..You need cooldown but you don't have to get the boots.
This thing out performs it.

build:
Last item. 
While MF does move pretty quickly, the synergy here actually comes more from the two ults being chained together. If you can communicate well with your ADC and manage to catch the opposing lane in your ult, MF can do massive damage with hers, since they are stuck. It's pretty easy to type a quick "ult" in chat, as you don't really need to worry about stopping to do so, but even so, I would recommend being in a voice chat were possible. This is probably one of my favorite synergies.
Draven is a great synergy because of your W passive. Normally, Draven likes to build massive damage, and you can help him with that by building massive damage. He gives you a lot of AP, and you give him a lot of AD. Your heal is very useful for keeping him in lane, especially if you can take guardian and he takes Fleet Footwork. Guardian will allow him to get in and win a good trade, while Fleet Footwork will allow him to heal more and retreat fairly quickly. The amount of shields and healing you will be doing is gross, but very useful.
Although a lot harder than a lot of the champions listed before, Twitch can be a very useful Synergy, but you HAVE to communicate well. I would say this mostly falls under a mobility Synergy, but you can do a lot more with it because of his invisibility. While you are attached to an invisible Twitch, you are invisible as well. This can make for a lot of good, unexpected, 3-4 man ganks. On top of that, if you are able to land your q, Twitch can follow up with a good W, making for a massive slow. His ult is also a really good combo with yours, as you snare the entire team, he can follow up with really good damage with his ult penetrating each one of them. As I mentioned, you HAVE to communicate well with your ADC for this synergy, because if you are not on the same page, throwing out a Q instantly gives away your position and puts you both in a bad spot. You can also end up wasting a lot of ults if he doesn't follow up with his.
A lot of people see pull champions as a good counter to Yuumi, but the only one I've really had trouble with is Blitzcrank. It's really fun to mess with an enemy Pyke by intentionally going in a bad spot and watching him waste his Q as you jump back to your ADC. To put a cherry on top of this cheese, your W does not cost any mana, while his Q is one of his more expensive spells, and has a very long cool down. You are usually safe from actually getting pulled, since his q has a very long charge up time, and initially doesn't even pull you, just does a bit of damage. He becomes an easy target to poke, since he is always trying to get around minions to pull your adc into a bad situation. If you play it correctly, you can prevent him from healing as well by warding everything. Assuming that he does not target you, abuse his negligence and just constantly auto attack him. He will get lower than you might think, plus it gives you a nice shield.
and every so often