>>717572591
I really don't think there's anything wrong with that, especially when you already have people like Han Solo getting the drop on Vader in A New Hope or Luke - someone who isn't even fully trained - being able to outsmart and overpower him at the end of it all.
Vader is incredibly strong but he's not supposed to be infallible. Star Wars is a setting where there should be characters that can rival Vader and the Emperor and Grievous fits that role nicely since he's specifically designed to be a counter to Force users.