This manga is simply bad. The success of the series is probably due to marketing.

The characters are so trivial. The villain are bad because they born bad. that's all. The heroes are good because they are good.

The story is always the same. All the narrative arcs are just the same. Each arc is composed clearly by litterally the same events.

The powers are nonsense. There are the devil fruits, ok, but everytime happens something that has nosense. For exaple Rufy is made by rubber, ok , but then he is poisened and he simply doesn't die... And the manga is fullfilled of stuff like that. Some villain shot at Zoro (that has no powers) and nothing happens. Sometime really random power ups happens without any sense. For example (spoiler) in the fight Zoro vs Kaku Zoro, Zoro pulls out of nowhere 4 other arms, 2 other heads and 6 other sword from nowhere.

The fights are bad too, just because the main characters wins just because they are the main characters. The are weaker than their opponent, but at the end they win... That's all.

Even the artstyle is bad, it's raw and caothic, and you can often find some wrong proportions and prospectives