Yuumi is hard gated by the best person on her team. Once the laning phase ends she's going to be on the person who's doing best to turn them into a juggernaut. I've played her quite a bit lately, and I can say hands down that she can make the person she's on very slippery, as well as give them massive chase down potential. Plus she gives them additional safety when moving close to bushes because she can scout them out with missiles.
Plus she's relatively safe for a support. While you do die some by people who go crazy not thinking that they're giving up the double if they go down, you can often jump from person to person in big fights to avoid dying. The enemy can't dive you like they can soraka, where trynd will just spin in and 2 bang you. You can also often read the situation and if your ride gets caught she can bail off and run to safety while they finish the main person off.
Plus she can absolutely frustrate some champions to no end. It's easy to jump off to block a cait ult, blocking a zoe bubble has no risk (fun fact, you can steer your missile while asleep), and while it's risky I've jumped off to block a pyke hook because I can immediately jump right back on.
I think one of the understated things about her is that she has an economic advantage as well. Most support dream of first back with 825 gold for upgraded support item + boots + pink, but she gets to skip boots so her ideal first back looks something like upgraded support item + pink + 2 faerie charms (775 gold). A lot of times this makes a big difference because coming back from the first back she will often finish stacking manaflow band, so at that point she can heal on cooldown to deny poking her adc. If you figure in upgrading boots she has a 900-1100 gold advantage, which is half an item for a support. All this without giving up any real mobility.