Yumi willl be the Shen/Ryze of supports
Her ult is like a big Taric E which can be wasted by teammates not going in. Her Q can be unusable with bad adcs. Your teammate doesnt know your passive or W cooldown and you cant even call passive cool down.
But voice chat and practising together a lot deal with these problems. She will be pro play only eventually so if you like the champ play her now.