I think it was imperfect do to the fact that what she tried to do WASN'T reincarnation, it was a complete personality take over! Like in "Otome Game no Heroine de Saikyou Survival" where the Overarching Antagonist of the story (even though she was the FIRST in a sense) tried to "change her fate" thinking if she used her crystal on the FMC, she'd forcefully be reincarnated as her, WHEN EVERYONE INCLUDING THE STORY PROVING IT shows that all that would have done is force HER MEMORIES AND PERSONALITY as in a COPY not the REAL HER into the mind of FMC! Making FMC THINK she was her! But it would not BE HER! Like thinking if you have a clone and die, you will be the clone instead of you know "dying as yourself" and the clone living life believing they are you from that point onward, cause in a sense THEY WOULD BE YOU SINCE YOU ARE KIND OF DEAD
She was more TRYING to posesss the loser, compared to how MC was kind of living within his current self sealed till memory unlock combined them! He didn't STEAL his body, he was already there, it's just much of his personality traits passed onto his current incarnation, and having a weak will, meant his stronger will took control! In a sense it's not HIM per say as he was, just his new self with his old memories! It also is likely her STUPID "Erase Everything" was literal and start erasing even her COPY!
It's like that Death Ability Manga where one of the Antagonist's thought to have a clone of herself confront MC to try and assess how to counter and deal with him and his ability, only as soon as she got control of her original body after the clone she was piloting is killed, she drops dead cause turns out his death ability DOESN'T kill....it ERASES EXISTANCE meaning just PILOTING her clone body meant he KILLED HER and that moment of life she had was just the last silver of herself still being ERASED! She lived long enough to see herself die just like that LOL and this feels the same!
The only annoyance was that her attempt to steal the losers body and his desperation to get something out of this affair allowed SOME of her power to be grab-able! But I doubt he'll be TOO dangerous! More an IDIOT WITH POWER dangerous then "he might make something of himself" type of dangerous because he grabbed power he hasn't earned! Of course he'll be LESSER then her! Despite her own shitty behavior, she still EARNED her power!