The purpose of this message is to temper expectations and to reduce disappointment, so that the viewer can enjoy the movie for what it is.
I think this movie is not really a One Piece movie: it does not capture what the manga or TV series is about or their spirit, and does not capture what makes them good or what people read and watch them for. It makes mistakes in areas that the series excels in and does things that are unprecedented in the series. It is instead a movie about One Piece, or a One Piece themed movie. I think you should see it as a 'fan made movie' or perhaps a 'fan service movie' or a film maker's (who is not Eiichiro Oda) original take on One Piece.
Since the movie is for 'fans' of One Piece and since it is not a 'One Piece quality' movie, I think generally speaking people unfamiliar with One Piece should not watch this movie.
I think the movie is still worth watching for those interested in the movie. What I found enjoyable in the movie are the very fantastic and beautiful songs and accompanying visuals in the movie, the original ideas or the original take on a One Piece movie, and the interesting and powerful set up or start. I would have rated this movie an 8 if the marketing surrounding this movie didn't present it as anywhere near canon or authentic.
Expect cringe AND enjoyment (at the same time). This is a movie you do not want to be canon in any way, because the way familiar characters act in this movie sometimes appears unrealistic or at least questionable. Perhaps the right expectation is to expect a nonsensical spectacle, because my constant cringing in the second half of the movie did lessen my enjoyment quite a lot. If I knew in advance that this movie was not to be taken seriously, I would have been more mentally prepared for this movie.
To give you an impression of what kind of person I am, these are some series I have rated highly in the past: 'The Sandman', 'Violet Evergarden', 'Vivy: Fluorite Eye's Song ', 'Great Pretender', 'Messiah', 'The OA', 'Sense8', 'Beastars', 'Dark', and the movies 'Interstellar' and 'Arrival'.
