It’s a 6 and a half or 7/10 for me. Brilliant art and interesting worldbuilding concepts, but the story, characters, and writing themselves aren’t that good once you get past the funny absurdity of “this is how America looks to other countries”.
It’s like a combination of Sand Land, Trigun, and One Piece but it can only actually live up to those three titles on the visual art front.
I wasn’t enough of a Boichi fan going in to recognize him but it seems he’s a One Piece artist as well as the Dr. Stone artist and the author of Sun-Ken Rock.
I know of Sun-Ken Rock and like that series, Marshal King has fantastic art but the writing is lacking.
Marshal King really could have benefited from a full time writer to refine the ideas Boichi has while he oversees the story and sticks to art and concepts because that seems to be where his strengths are.
TL;DR: 7ish/10. Not bad but you won’t miss out on anything by skipping this title. Heavily carried by some great art. Writing is for better and worse rule of cool. I’ll be very surprised if it makes it past the notorious 30 chapter cancellation rule Shōnen Jump has.