Why is she so low tier???
Her normals are very slow with medicore range compare to the rest. very little combo ability outside of chains(speed), links, and cancels.
Her commands are even worst as none really stand out from other cast. She has reflectors/counter that has minimal usage out side of reflecting projectile, it also auto counters all mid& high base attacks for knockdown (usually repel a better option), Both her projectile appear identical except
has larger start up and recovery. Shoryuken is.... weird as the animation show her launching an ice pillar of sort looking like a projectile attack and yet opponents can still repel it and since she stays on the ground she is ground repel exposing her to many damaging combos. She has a run slash that pretty much useless out side of cancel into her DM (Pow) and meaty scenarios ( it does seem to has catch animation but have found little use). Her DM has very little application besides roughly extending combo potential. Her SDm is an unblock able which is nice except it can be interrupted easily by some cast, and avoided if not done properly.
Between all these. She consider the strongest in Speed mode has their were she gets more of pratical combos and some mix up. Pow DM cancel and SDM have limited application but makes her offensive slightly better. EX gives her the most damage and tools but her general gameplay is weaken by worst health and slower meter gain which is usually not worth it except for Hail Mary's.
With all this the only way for yuki to be successful is have outstanding knowledge of match ups, and the player them self...simply being no errors as Yuki has a hard time switching momentum.
anything she can do, majority of overy one else can do better. Buty also I think the main thing is that while she looks like a shoto on paper, I think she is grappler in disguise as her best tool is a command grab which open up many opportunity's for her. For the most part she isn't garbage character material but she requires mastery of fundamental just to be an contender.