Issues that need to be addressed with Yoimiya's kit in order for her to be viable:

1: Auto-attack playstyle but auto-attack can miss
Suggested fix: significant tracking on projectiles (even this is problematic as it just makes her inability to target specific enemies worse.
2: Auto-attack soft-lock-on dramatically lower than her effective range
Suggested fix: Either add a hard-lock-on to the game or allow Yoimiya to soft-lock from low orbit
3: No AOE potential
Suggested fix: all skill-buffed projectiles have either a significant explosive radius with no damage falloff or multi-target penetration
4: Burst can miss.
Suggested fix: snapshot burst to hit on button press, do not repeat the mualani projectile shitshow
5: Burst exists to give a sub-dps more damage
Suggested fix: just make it a big pop of damage with a large radius you fucking idiots
6: combo damage is heavily backloaded as a response to 1.0 combos being mostly optimal on combo cancel (something that still happened next patch with Raiden)
Suggested fix: Add Dodge Offset, a property of combo-focused characters where their combos continue after they dodge but not after they are hit (Itto has this property, they know it exist and are capable of using it)
7: generally poor scaling on her Skill
Suggested fix: decrease damage bonus from NA talent and increase skill talent bonus. Or just raise both because she's already undertuned.
8: charged attack gets worse during skill uptime
suggested fix: don't do that

miya