There was one chapter (I forgot the chapter #) during the dressrosa arc that was literally nothing. It just showed what was happening at different parts of dressrosa and thats about it. Plot didn't move, very little dialogue. But it would flow well if you were marathoning the manga.
None of them are completely useless. Some are indeed a bit slow-paced and boring, but they add something to the story, so that doesn't make them useless.