Yeah, I figure there are some cases where it's just like, a straight-up one-to-one thing where whoever or whatever is doing the possessing completely replaces the personality of the host.
[ like when maya channels people, though he has no idea that's actually a thing! anyway i feel like this must have happened at some point in his old 60s/80s anime in a stupid way ]
But it does seem like just influencing rather than overwriting is more common as far as these things go.
It's nicer to have something to blame besides yourself. And given that we know people in these situations just go somewhere else to deal with the aftermath alone...