As a long time fan of the show, I appreciated the film's typically exceptional character development, as well as returning to the interesting character dynamics that subvert character tropes.
Spinel's character entirely stole the film with her presence. But ultimately it's not what a Steven Universe film needed to be, at least not in this point of the story.
It would have been a great side-story or material for a later film... but the film barely addresses the developments of the previous arc. I.e.:That climax the entire series was building up to.
For example, what happened to the empire? They say it radically changed political systems... but does it show it?
How can it be a radical change, how can the empire be dissolved if the diamonds are still in place?! Weren't they tyrants who repressed the masses? Why are the subjugated gems cool with the diamonds still running the planet?!
Sure, there's a couple of throwaway lines: Yellow said she dissolved the army and white said she started treating other gems as equal... but that's nowhere near enough.
But even accepting that the entire social system of the planet was turned on it's head and that the gem population would accept the tyrannical diamonds place as rulers... that still doesn't mean there wont be unintended conflict and repercussions of the dissolution of an entire interplanetary governmental system!
Just look at history for examples of this, the non violent independence of India against British rule lead to millions dying and being displaced as a result of adapting to the new political order of post-independence India and Pakistan.
Of course I don't expect high level politics from SU, especially not condensed in a film format, but my suspension of disbelief can only be pushed so far. A peaceful transition from a million year old authoritarian order to liberalism however crosses that threshold for me.
A much more important film to make would be about the repercussions of Steven destroying the empire, maybe a civil war between factions within the newly established system.
Maybe getting Steven to see the consequences of his actions and thus grow as a character, instead of just suffering through the consequences of his mother's decisions all the time. Show that Steven is also capable of making mistakes etc.
That being said, there are some glaring issues with the film, even independent of the larger context.
For example, the first 10 minutes are sort of wasted just re-establishing the backstory and developments of the main series. (Which is another reason why the movie would have been better as a standalone -a 10 minute long recap wouldn't have been necessary)
Secondly the ending was a complete anticlimax, they conveniently ended two major issues: The diamond's clingy attitude and finding a friend for Spinel. Which I agree with @DokiDokiTsuna was a relationship that feels absolutely forced.
So effectively the film redeems all the major villains and let them get along and have a happily ever after? What kind of moral is that?
Still, It's more of what I love and expected, so I'm happy with what I got, in spite of it's multiple issues.
Personal score: 8/10
Objective score: 5.5/10