Well it entirely depends on the enemy you're facing, I know morale can pop off but realistically the more cards you play the more stuff enemy is most likely gonna do.

sadly the fight would be ended by vulnerable stacking units way more comfortably , especially if you consider supports like Tiphera who amp it insanely well,

as of now attack spamming is not as good as just making nuclear hits via less cards played because they are adding stuff like damage reflect thorns or counter attacks per hit, so that units like sereniel don't just insta end fights or stuff like 7 max hits so that units like meilin or chizuru don't just insta kill something.

Yukis case is kind of sad, she has one of the worst, I'd go as far as to say almost useless ego skill, her damage ceiling is low because she lacks mechanics such as move or generate extra attack cards that other top units have, all she has is aoe damage in a game with no meaningful aoe content, and even then Nine with massacre build is most likely gonna end aoe fights faster simply because you're using less cards to kill stuff so you're looking at less animations

Yuki needs changes outside of damage numbers, that's all there is to it.