The movies and show never really made clear whether Grievous(or at least the brain and other bio parts of him) were force sensitive or not. Clearly with Vader a force sensitive with cyborg parts is still capable of jedi fightin’.
If he wasn’t, and he was just using machine precision to twirl those lightsabers and kill all those jedi…well shit why didn’t they make a bunch of droids and arm them them four lightsabers? Palpatine could provide the sabers.
There was a theory going around before the movie came out that Grevious was a rebuilt Darth Maul, which would probably have worked better than what Lucas came up with
Even if he’d been force sensitive before, the process of turning him into a cyborg would have wiped out his force sensitivity. He was just a brain and a few organs. Vader had much more of his original body than Grievous.
It’s just another inconsistency of the Star Wars universe - they have several million incredibly crappy droids that can’t do anything right and exactly one droid capable of fighting four or five jedi at once and killing all of them.