Alright, I’ve decided to make a list of the issues the AI has after all (the issues that I know of, anyway). The main issue is that not all of the characters use all of their skills, not even at Expert difficulty:
Kotatsu: Doesn’t use her Wall Jump
Cali: Doesn't use her Counter Stance
Ginny: Doesn’t move while carrying a ball she’s caught. Also if she catches a ball, AI opponents don't move until she releases it, when it'd be more beneficial to try and stay in front of her.
Menoko: Doesn't seem to know she can block and parry while moving with only her momentum, making her very susceptible to chip damage and Syrhild's Meteor Smash.
Suji: Doesn’t use her Foxfire
Jill: Doesn’t use any of her abilities
Smaller general issue: At the beginning of a match at the East Birthday Street stage, the ball starts in a position where it hits the players’ pads straight in the middle if they don’t move, so at the beginning of a match there the AI will just let the ball bounce back and forth if nothing changes its trajectory.