Or rather, its not recommendable if you are good at using ReimuB since her damage output is better.
(for extra)
Stage: advantage SakuyaA. Considering I fail at this, it may be worth the switch.
Chen: advantage SakuyaA, due to Blue Oni Red Oni. ReimuB has the advantage on the final spell, but that's easy.
Other stage: Still SakuyaA. I fail this less, but ReimuB gives me issues at the spam-->streaming
nonspells(all): very slight advantage ReimuB, because of the 5th nonspell. SakuyaA always hits though, which is why it's so slight.
1st spell: SakuyaA, because always hit.
2nd spell: tie.
3rd spell: advantage ReimuB, still not a threat.
4th spell: The bane of my existence. advantage ReimuB, unless Ran plays BS, then SakuyaA (a strategy for this that doesn't require any luck would still be appreciated)
5th spell: advantage ReimuB, although I capture it anyway.
6th spell: completely trivial. ReimuB is faster, but it doesn't really matter.
7th spell: tie.
8th spell: tie.
9th spell: advantage ReimuB. Probably Sakuya's biggest obstacle.
10th spell: easy easy easy. tie.
So in general, ReimuB is better for bosses ,but in most cases I'll do just as well with Sakuya-and it keeps me from having to grind the stage as much.