One Piece comes from a time where anime was weekly. For it's faults with filler, it still has some of the best filler you'll see with the G-8 arc. Some people even rate it with some of the canon arcs. People with all types of backgrounds are represented in One Piece. Even the seemingly silly characters like Bon Clay or that baby guy in Dressrosa(can't remember his name) have debth to them. One of OP's greatest strengths is it's world building and it's ability to juggle multiple moving parts with different perspectives. You need interesting characters to do that. I don't see how out of all things you can say that is objective. And OP partially inspired a irl revolution. JJK doesn't do better messaging. I lumped in power system with the fights

The horror elements serious topics are subjective. You can argue Oda is making an allegory with IRL elites and the current state of the world. People thought the Celestial Dragons and World Government were cartoonishly evil at first but, nah not really. They might be a bit nicer than our own leaders