I've always been a bit reluctant with the genre Musou, I saw them as a button masher and the truth is that I didn't like them. However, this year I've been encouraged to play several, including one of the pirate warrior saga and the truth is that they are really fun. I don't know why, but it has a certain charm to burst those giant hordes with your attacks haha.
Although I played the sequel to this one, the 4, which covers a bit more of the One Piece story. Highly recommended too.
They are button mashers but as you said, they are fun. There's a lot of content in 3, I haven't played 4, but with less arcs being featured, I dunno if it'll be as big.