The strongest element in this movie is Rintaro's direction, and if we judged only by visuals this would easily be an 8/10 or above. But as a stand alone story and part of the 999 franchise it's pretty atrocious.
As I mentioned before it makes the same weird decision from the previous movie of taking out everything that 999 had besides character designs. Between 2 movies we get none of the character development or plots of the original. Emeraldas even attacks Prometheum and at no point it's mentioned that she's shooting her mom (ep. 12 of the tv series explains that she's Maetel's sister, for the modern OVAs it's put as a given that you know this even though the movies go out of their way to ignore it). A story about travelers was turned into a story about freedom fighters. It's like taking Kino no Tabi or Mushishi and turning them into, well, weak Harlock spin offs.
There are other weird pointless connections like Prometheum being made Queen Millenia, there is no attempt to connect with it or make it fit in any way. Being a frozen planet, crashing with earth or any other planet, being queens for millenia. There's nothing besides names. Just like 999.
As it's own thing it meanders a lot. There are three different instances where Harlock just appears and saves everyone because he happened to be there, including the one time Maetel was trying to do something. With this plot it makes no sense to have a train since it doesn't have a set track to follow and they have no passengers to drop or pick up, it's just an aesthetic element. Everything about Maetel makes no sense, no matter if you know the original plot or not. She just says things that couldn't make sense and don't explain things she should want to explain.
And final spoilers: The ending is the same thing that the series did, and the previous movie, and the OVAs, and I'm sure the planetary only shows have the same. Just watch the tv series.
