The only irreversible effect is the mental invasion, which is also the easiest to deal with if they don't have glaring mental issues.
And honestly depending on the person he might not even use it.
And Ivan, most of the really super high level people are hard to turn. And Evil does not mean "KILL EVERYTHING" for everyone. His goal is not to kill people, just to make people be themselves (which is in his eyes their repressed evil).
Honestly the highest level person easiest to turn would be Jubei, cause he got shit in his closet man.
And Chris you hit the nail on the head. He always tries and make people evil. But it is a gradual process performing small acts of evil as it integrates more into you as time passes. The temptation to preform evil acts is always there, but it is up to the person if they will act on it.
I mean he can cause an abrupt change under specific circumstances. But that not his style. People have to chose their path themselves, he just helps them along the honest one, giving them a little nudge in the right direction (Disclaimer: Nudge may or may not consist of causing intense mental and emotional trauma, your mileage might vary).