1. Her auto tracking and range is terrible (global archer issue)
2.  
3. Her strongest attacks are locked behind late animations in her attack string (N3, N5 especially), and they're very hard to execute due to the fact that she needs to dodge a ridiculous amount due to having some of the lowest hp/def/interruption resistance in the entire game
4.  
5. ICD makes vape/melt comps extremely inconsistent and there is zero reason to use her over Xiangling, Hu Tao, or even Diluc in such comps.
6.  
7. Mind-bogglingly bad burst and effectively zero AoE
8.  
9. Thus, her most popular comp is Overload - however, due to the nature of Overload and the bad tracking and slow velocity of arrows, her Overload procs literally make her following shots miss. Mono-pyro is playable, but takes away two of the best supports in the game (Bennett, Xiangling) from your other Abyss team, and again - she can be replaced by Hu Tao, Klee, or Yanfei and the team would improve.
10.  
11. Her signature weapon (Pulse) and signature artifact set (Shimenawa) conflict with each other and her rotations. Using Q punishes you, as you now have no energy to use the 4pc effect of Shimenawa. This forces you into an E -> Q rotation, and the 4pc effect of Shimenawa makes it difficult to have her Q back for the end of her rotation in many instances. This is a big issue because both of her constellation and passive revolves around having Q debuff uptime on the enemies for as long as possible.
12.  
13. As such, many people skip RoS and her most common and optimal gameplay becomes pressing E and spamming left click. There's almost no nuance, little skill expression.
14.  
15. Yoimiya's issue isn't that she's weak, or underperforming for a limited 5-star - it's that her kit is thoroughly underdesigned, uninteresting, disconnected, and very evidently undertested. Whether this was a result of beta testers completely ignoring her in favor of perfecting the more-popular Ayaka or sheer incompetence of the testers or developers is irrelevant. Yoimiya's kit is arguably less fleshed-out than many 4* characters, with a disappointing amount of consideration for its actual gameplay.
16.