Well I can kind of guess why this can get a lower rating than the main BangDream storyline(?).
It is much more serious and solemn and less fun and funny compared to the main universe. But this is Roselia we're talking about, not PoPiPa, so......
But even with just the duration of a movie, we get a much more in-depth look of the characters' feelings, more than what some idol anime achieve in a whole season. The members of Roselia are not just some undefeatable oddballs, and this changes how I look at the characters in other BangDream media.
Overall I wouldn't say this is as enjoyable as the ??BangDream main story??, but this is great to watch as a fan. Looking forward to the other bands' episodes, and it makes me wish LoveLive did it even if just to extend the screentime of the characters.