One can argue that there isn’t much plot to this film but honestly, the direction and set design are amazingly good. The word I keep coming back to is “unsettling”. The whole film is unsettling. Even when we’re not in the Backrooms, the rooms and people encountered are all “off”, and it becomes increasingly difficult to see where the line between reality and the other world is drawn. For example, when Mary comes up the stairs at the end into the showroom, is that the real showroom or a copy? I couldn’t tell.
In short, I really liked it. And I think it’s been deliberately teed up for potential sequels.