It’s very simple. Law’s Ope Ope no Mi devil fruit grants him no cutting abilities. Law needs to be strong enough to cut and kill someone to perform his surgeries.
He actually would’ve killed Vergo with that attack in Punk Hazard but Vergo survived because he was in Law’s room. However, if it was Doflamingo in Vergo’s place, Law wouldn’t have split him in two because Doflamingo’s Haki is stronger than Law’s.
The ROOM doesn’t increase Law’s cutting strength, it simply stops the bloodshed and keeps split body parts connected in an invisible way. Law needs his swordsmanship and strength to slice someone. But people seem to believe that he can split anything in the room, which is simply not true.
If Usopp had the Ope Ope no Mi devil fruit, he wouldn’t be able to take Smoker’s heart because he couldn’t physically slice his chest open or punch his chest hard enough to pop out his heart.
Law can’t switch Kaido’s personality (or mind) because he’s not strong enough to cut Kaido’s skull and take out his mind. He can only teleport his whole body without cutting it in any way.
Law’s devil fruit ability isn’t as overpowered as people claim. Some even claim his devil fruit negates durability and just cuts whatever there is. But the truth is, he cuts a lot of stuff easily because he’s just that good of a swordsman. And a ‘D’.
I’m saying this to also make people realize how good of a swordsman Law is, that he makes people think his strength comes from a devil fruit.