@Masaki HarunaIf you're looking for a limitation then surely the Kitsune assimilation should have a hard limit, right? I mean, it was another person at some point, and I doubt just eating them would give you all of their powers even with magic. You could just say that the Kitsune wasn't fully assimilated so you won't have full access to its powers or something like that.
Alternatively you could say something like; Kyou didn't completely assimilate the Kitsune so her body is still partially human. Using the Kitsune's abilities or the Kitsune's magic too much will take a toll on her body as those powers weren't meant to be wielded by humans to begin with.