Some Fridge Horror I've just realized about this whole "Perfect Possession" thing...
In the ending of the Extra Stage of ULiL PS4, Reimu was shown to be suffering the excruciating pain that came from Mokou's regenerative powers as a Hourai Immortal. And given that Mokou pretty much DIED several times and regenerated just as many times, this must be one hell of an excruciating pain for Reimu...
Worse yet: the ending also looks to me that whatever injuries inflicted upon Mokou that isn't fully healed, Reimu KEEPS them! Thankfully the injuries kept are not bad enough to be fatal, but imagine if it was something worse...like shattered ribs, broken spine, amputated limbs, having a large fraction of the head blown off... And we all know that Reimu's death will lead to the collapse of the Hakurei Barrier... *Shudders*
Even worse: before the fight began, Reimu immediately blames her losing control of her body on possession, shortly before literally turning into Mokou. That line seems to be implying that during the entirety of the fight against Mokou, Reimu is FULLY AWARE of what is happening: Mokou giving the Spell Card Rules the middle finger and tries to kill Reisen - a friend! And Reimu couldn't do anything but be forced to watch "herself" try to kill a friend using one excruciatingly painful Kamikaze attack after another, and said friend killing her back several times in self-defense!
I would be extremely surprised if Reimu didn't develop worse PTSD than Reisen did from the Lunar War after all that...