The Backrooms movie felt like it was not really about backrooms at all.
I have always been a great admirer of the liminal spaces and cosmic horror genre in whichever formats they appear. Consequently, I had really enjoyed Kane Pixel's Backrooms series on youtube which supposedly served as the conceptual and thematic foundations for the movie. I loved the series primarily because at least in my opinion no other piece of work has existed up until then that comes close to it in terms of treating the concept of liminality and "uncanny structures" as the core driving force of its story or theme. However, having watched the movie it certainly does not seem to be the case for it.
Here's what I think are the shortcomings of the film:
1. The backrooms is only secondary.
In the movie, the backrooms is portrayed as a tool to understand human trauma, and not as the primary subject/focus of the film. Now it certainly can be argued for arguments' sake that the film does not explicitly set out to do this. But everything surrounding from how Clark discovers the Backrooms and later the therapist gets to see its operational properties says otherwise. The film even opens with a monologue about "habits", "loops" and how the "untrained mind" eventually finds itself trapped because of them.
As the movie progresses further we get to know a lot more about the characters and the positions in their respective stages of trauma. Whether the film gives us adequate information about the characters or if it is efficient in communicating them to its audience is a different matter in and of itself, however the deeper problem from having set the backrooms as a "means of catharsis" for the human protagonist(s), is that as we start to know more and more about the characters in the film we inadvertently are knowing more about the backrooms itself, which I think goes against the very concept of liminality and the backrooms.
The youtube series avoids falling into this trap by literally and figuratively having no protagonist. Or a protagonist that serves as a blank canvas. In the series he is a random youtuber who stumbles upon something artificial and exotic. And he goes in to explore this vast unsettling place in the middle of nowhere and because he carries absolutely no emotional baggage or trauma the audience has zero expectations or inclinations to paint the exotic and unsettling backrooms in any manner whatsoever further personifying it.
2. The backrooms do not feel endless
Maybe its because there's very little actual exploration of backrooms but even then there isn't that sense of melancholy and hopelessness that comes with the feeling of being trapped in it. I think this partly has to do with the fact that Clark can always find his way back to the store and the only reason he is "drawn" to explore it besides its absurdity is his own traumatic past. But a big chunk of it I think comes down to them going down the "generative ai" aspect of it all.
Newer sections of the backrooms feel very different from the previous ones and their boundaries are quite sharp and because a significant chunk of the exploration happens while the protagonists are in distress or when they are being chased by the pirate make these transitions abrupt and jarring. They break the immersion of being in an endless repetitive maze. Though whenever they switched to the found footage mode the aspect ratio certainly helped to curb the generative ai shenanigans that felt a bit over the top at times. They seemed to rely a bit too much on these oddities to bring the unsettling nature out of the backrooms when in fact the liminality should have brought it on by itself.
3. Changing protagonists
For the the better part of the first half we have the POV of Clark the furniture salesman. Then for a little while it shifts to include Clark and his assistants as they set out to "research" the backrooms. Then it abruptly ends and shifts towards the therapist. For any other genre of horror this isn't a big deal but in the case of backrooms this prevents us from witnessing the effects the backrooms have on a singular individual over the course of a long duration of time. Everything seems to happen in a matter of days even though Clark has spent quite some time down there. The final act with the therapist starting from the moment she enters the backrooms to the finally being rescued by Async feels so rushed and seems to happen during the course of a single day. Now mind you this is another person who is having to go through the introductory phase all over again to repeatedly see the same cycle of "awe, horror, curiosity...". Instead if we just focused on the POV of a single protagonist we could have avoided repetitive emotional responses, have prior reference to know how the protagonist reacts, plus would have forced the makers to explore newer sections.
I have many more smaller details to nitpick but many have already been repeated in some posts here, one good one being how they changed no-clipping to a door essentially making the backrooms an extension of the store. I think this really had an effect in making the backrooms feel a little less unnerving than it could have been, since in the back of the mind there's always an escape route back home.
The movie certainly is unique in exploring a novel concept and an alternative form of horror so I will give it that but it certainly feels like the uniqueness ends right there. The concept of backrooms was used as a hook and an entry point while still relying on tried and tested tropes, human drama, malevolent/dangerous entities, etc etc to elicit horror. The backrooms served merely as a host for these actual agents of horror and any effect it had by itself in producing the same is trivial and secondary at best.