It's not that it steals from Star Wars, the movie borrows from a lot of other sources. Many writers and directors do this, though this is done sloppily in Rebel Moon. It has some John Carter of Mars, an homage in makeup design to Mad Max Fury Road, weapons from the 40k universe, its a drawn out version of the first 10th of Seven Samurai and many more.
The real issues are the pacing is odd. Introducing characters with little to no characterization or understanding of who they are until an exposition dump towards the end. The fighting uses slow-mo cinematography in places that destroys the momentum of the action (they also clearly didn't use enough takes as some of the attack swings are way short). The CGI is really good in some places and just awful in others. The ships have little to no weight to them. The capital city of some king looks more lifeless and blatantly computer generated and fake than the world of the original Tron movie.
The script is terrible. It lacks focus in theme, as characters bring up honor, revenge, pride and other motivators but often what they are clearly trying to build up they destroy that idea immediately after. One instance trying not to spoil is a character gives a speech about how you should not use vengeance as she fights an entity that is trying to get revenge, but at the same time this is a recruitment moment where she is teaming up with people trying to get revenge. It's so clunky, relying on exposition to cover for a lack of motivation delivered naturally by the characters.
The acting overall is not great and this clearly is influenced by the direction which is worse. Some characters accents aren't consistent, and the lines they deliver are often stilted or given long pauses for no reason. Just eating up time in a movie that is already too long for the lack of story it has. Some of the characters aren't even given a personality/character trait beyond drinks alcohol, or acts cool and emotionless until you've already given up on being entertained and are now dreaming of doing the dishes. It also falls onto stereotypes which are tired at this point.
There is some good to it. Some of the creature designs are cool, some of your favorite actors got a paycheck. The actor for the villain does a very good job 95% of the time eating up scenery and being someone you root against. The soundtrack is decent. There are times when the cgi is good. Overall though this is a terrible movie. I will say it is worth "watching" or being as background noise when folding laundry or doing the dishes, making food, etc... It is not worth your undivided attention.