So as you've noticed by the trailer, the film has a unique way of presentation, using the old-school aspect ratio. Its not perfectly clear whether they used real film stock, or created it digitally, but as someone who worked at a film archive, I can attest that they did a pretty darn good job. But is it scary? Is it really like the critics say, that its the best found footage film in years, being a ode to The Exorcist and Rosemary's Baby? Well no, its not anywhere close to those masterpieces, but I agree its much better than the found footage garbage we so often see. With a boat load of Hollywood found footage trash, we also get a wave of films made by anyone who has an iPhone, and the vast majority of them are terrible, and while Devil's Doorway is not amazing, its certainly to as bad as I'm used to.
But was it scary? Its hard to say yes or no, I would rather say it was hit and miss, some of it got me, some of it didn't. The atmosphere also appeared and disappeared, sometimes it was effective, sometimes it was either not there or was not as effective. Its no masterpiece, but it most definitely had some cool stuff in there. The aspect ratio adds to it, the story its based on is an interesting one (surprised it has been done to death), and some of the plot points, shocks, and scares were alright. Again, alright, not perfect. So don't go in expecting a masterpiece like some critics say, or are paid to say.