In my opinion, Oda's art has declined significantly in the past couple of years. Onigashima is the worst the manga has ever looked in terms of art and you only have to compare it to early Wano. I do believe that Oda's art has gotten better over the years (just look up the drawings he does of Luffy for the One Piece magazine), but I feel as though he tries to cramp as much information as possible into a single page, causing the art to suffer from it a lot. Another problem with this method is that the small panels he draws nowadays leave less of an impact on readers - they become less memorable and thus forgotten about, which, I think, largely contributed to Onigashima being as underwhelming as it was to many people.
(This is just a tiny example. I'm sure you can find many more throughout the series.)
(This is just a tiny example. I'm sure you can find many more throughout the series.)