The premise of One Piece is quite simple. Oda created a story where the main characters can travel to islands and that island’s arc is a completely new genre. Oda can go from high seas adventure, fairytales, Shinto mythology, and science fiction without blinking an eye. We are supposed to learn about the world through the SHs.
The problem is that the One Piece world is so big and Oda only wants to focus on Luffy. Luffy is a great character during the pre time skip, and I still like him post time skip too. When a world is so vast, a writer needs to show multiple perspectives. Oda refuses to do that. Instead everything is through Luffy’s perspective and he’s a very straightforward character.
Furthermore, Oda has a habit of resetting his characters each arc. It’s like his world is expanding as his readers got older, but Oda refused to let the SHs grow. Instead they react to things the exact same way and it’s usually on a shallow level.
Basically Oda created a world and story that would have been more effective if he focused on multiple perspectives.