After playing five games as Yuumi and not getting a leaver buster, I can now impart my ancient wisdom to all aerie mains.
1. Rush Mejais - You shouldn't die that much because you are always attached to someone
2. Build only AP - her early game is dreadful and her late game gets even less meaningful if she builds pure support items. A 30% increase on her healing without much AP gives minimal yields on her heals and shields. Yuumi can actually deal a fair amount of damage with her Q and R, give over 100 AP or some amount of AD in adaptive force, and can heal lots of damage if she builds only AP. Late game her Q has a ~4 second CD and and that can really break a game. I recommend Luden's Echo and Rabadons after your Mejais.
3. Don't afk attach as Yuumi! You need to dettach every now and then to proc your passive so you can reattach and grant the shield to your allies. This is also useful for her ult sometimes because your teammates have a tendency to not play off your ult often enough. You should be weaving through your allies to get an advantage of positioning and healing. You should also detach from your allies if they are definitely going to die because it gives you a head start in running away from the enemies.
4. You are better off maxing her W second instead of her E because the E has pitiful returns on leveling and the amount of AP you receive along with the adaptive force you give goes up from 5% (which is pretty much nothing) to 21% (which can be around 80 AD or 150 AP to them and plenty for you!) and if you have a Jhin on your team, you will be reaping the benefits a lot more with that
5. Flash might not be too useful on Yuumi because she doesn't do much if she's by herself with her lack of mobility without any ally champions nearby, so bring heal or some other spell to make a difference
6. Glacial augment + glp is a good synergy on Yuumi because it gives her something to do and the slow fields will add some peel/chase/AOE that she desperately lacks. Want to take it one step further? Bring as your secondary runes ingenious hunter + ghost poro. Your items and trinkets will get a 40% cdr when fully stacked and your wards will generate ghost poros and improve your vision game. If you choose this route, don't pick up Ludens Echo as well or you're giving up slots for other items with unique abilities.
7. Don't buy boots until much later in the game because these boots aren't made for walking if you are always attached to someone
8. Don't play her in ranked please! She has a surprisingly high skill cap and you are not ready to unleash her yet. Also Riot will probably be buffing her a lot in the meantime because no one knows how to play her well yet.
9. Don't take her into a solo lane unless you are super confident that the rest of the lanes don't need a more traditional laner. She does not have any farming abilities other than her Q that is single target and easy to dodge and her ultimate, which means she will be pushed in and also she is one of the most fragile champions in the game. The only thing she can pull off in lane is roaming and funneling other champions.
10. Yummi takes turret shots, I know it says that for her W, but that means you cant sit on someone with your W and harass the enemy with your Q or R under their turret or you will be hit by the turret