Mugen Train solved the thing that held back Demon Slayer from being quite boring or frustrating. The pacing. Pacing in this movie was great, and maybe this series was just made for a movie format. Hopefully moving on since seasons are only 11 episodes long they've fixed their pacing issues. With that, the movie was quite enjoyable. They used elements that I'm glad were utilized with the themes they were exploring. Such as grief, proving oneself, and moving on from the past. However these themes are already preestablished as the entire character motivation from the get-go and really we don't learn or develop any character more than before. So as cool as the dream sequence is conceptually, it's not anything new or actually relevant at all to the characters or story. I really liked the first half of the film the most. Where the character interactions were much more delightful despite not being very funny. It was endearing nonetheless, especially the introduction of the Hashira, Rengoku. He's not the deepest or most interesting of characters, but he's got good enough chemistry to still be enjoyable to watch and not a pain in the ass like Zenitsu. The movie felt actually well put together despite its very basic flow and ideas. . .
Until the end. The ending of this movie was contrived and completely disconnected from the rest of the movie. It was necessary but the way they decided to push it was lazy and horribly executed. Without getting into spoilers its something out of left field that has nothing to do with the rest of the film, it isn't mentioned, foreshadowed, or even anything personally involved. It just happens. If they had better set it up as a plot point like the main villain of this movie saying, "There is someone else involved" or "I have to prove myself to that demon that I can handle a Hashira." Because that's where it falls flat, it's like as if at the climax of Dragon Ball Super: Broly they say, "No wait. It's not over" and then a whole new bad guy shows up to fight. It's such lazy work into a very competent plot that didn't need it. The ending was my main gripe with this film and why it's brought down even lower. It was fun but it sucked at sticking the landing at the end. This is also where the soundtrack stops being a highlight for me.
It is also insulting that this is the highest grossing Japanese film of all time, being just an alright film with no real message besides "believe @ yourself" which could suffice if that was ever a thing that the characters were struggling with, but they never did show any doubt that the characters had in themselves. It's a fine movie, but has almost zero real substance that furthers the story or characters.