Don't understand why this is so confusing for people. We literally see Luffy casually use it in Udon against fodders, how can questions like "has he mastered it" or "can he use it at will" still even be a thing?
Big whoop, he got hit by an attack while he was
angry, it's not like we've seen that happen to a certain 48 year old yonko first mate who had
way more experience in using FS....
Oh wait...…..