The other guy stating her counters is pretty much correct, but another counter to Yuumi is Soraka because of her Silence/Root, preventing you from W'ing back to your lane partner.
I wish I could give the advice I COULD have been able to give if Riot didn't butcher her into the ground, but she can still be useful.
Because of the nerfs, you're solely at the mercy of whoever you are on, because hopping off your teammates is just too risky due to the 5 second CD on W if you get hard CC'd. In Early game, you should atleast look for a suitable time to hop off your partner for a short second and use your passive to get mana back and hop back in as fast as possible. Make sure the enemy doesn't have or already used their hard CC before making the decision, because if you get caught, you're 99% dead.
Once you're in team fights, it's very risky to switch between teammates because if you get CC'd, you're 100% dead this time, so make sure you stick to someone you believe can dish out the most damage and carry the game. Don't even bother trying to use your passive AT ALL in team fights unless you're 100% confident you can make it back to your partner ALIVE. So in these cases, stick to someone you know will carry and win the fight, and keep them alive at all costs. It is OK to try to keep other teammates alive, just make the decisions wisely.
Because of the nerfs, it is much harder to be successful in lane, especially at the mercy of your lane partner. If they suck, you suck, it doesn't matter how good you are with Yuumi. Either communicate with your teammate and work around each other and the enemy, or pray that one of your other teammates starts carrying and stick to them instead.
On to roaming... NEVER EVER roam alone, because if you do, you are just a walking snacc waiting to be eaten by the enemy jungler. It's not a matter of WILL the enemy jungler kill you, it's a matter of WHEN. Don't take chances, stick with your teammates. It's OK to hop off and float alittle to ward a brush, but you must ALWAYS be in range of a teammate with your W, because you know what will happen if an enemy sees a tasty floating bag of gold. Roam ONLY with your teammates.
Make sure to use your ult when it can guaruntee a kill or a win in a fight, I'm not going to tell you exactly when or how, just follow your gut, just don't be an idiot like me. Just always use it in a teamfight for sure, it is Yuumi's strongest ability.
Builds? Rush 
, then go in this order: 
The last two items can be whatever you want I guess, I don't know the latest Yuumi builds these days, but this is what I've been following for a while now. Oh yeah, and take 
and
. You don't need flash or even heal anymore, since you can't teleport with your teammates now T_T
That's about all I got to give now I think. Been maining Yuumi since Day 2. Don't like at all how Riot has been treating her, but that's just what happens when you main 2018/2019/2020 Reworks and Champions, because more often than not, Riot WILL kill them as a champion.
But anyway, have fun and good luck out there. Have fun with any champion you want. [sg-ahri-2]
(Before the next changes come with pach 10.2, Soraka's a hard counter since her silence disables your W) 
top, 
mid, 
jungle, and 
adc
They do her job but better, unless you plan on playing the ADC and the support at the same time
support. its a lotta fun in a duo lane from the fact Xerath outranges almost all adcs Yuumi's W passive makes his power spike come far sooner and her E's movement speed makes it easier for Xerath to Q people while charging it.