I thought of another way to expand my review above without giving away the plot.
All films - but especially genres that deal with not just fictional stories but with entirely fictional premises - require a suspension of disbelief. Horror and sci-fi are on par as requiring the most, because depending on the film, the entire story - not just characters and plot - is based on things that the audience knows does not and/or cannot exist.
In horror films, the suspension of disbelief is almost universally required regarding the monster/entity, etc. Zombies - can't happen. Vampires - can't happen. Werewolves - can't happen. Escaping the dream world to terrorize teens - can't happen. Monster that comes out of hibernation once every decade/millennium, etc - can't happen. Ghosts - 99.99% of moviegoers have never claimed to see one. Psycho killers can happen, and I think that's why they've occupied a special place in the genre - because they speak of a monster that does lurk. But psycho killers that cannot die no matter what - well, we all know that can't happen.
What this movie does so successfully is that it requires you to suspend your disbelief in many different areas, but the "monsters" are the least significant of those requirements. They are the "given" in the story that is even suggested by the title of the movie. But it's everything else that takes you for the twisting ride as the layers of the plot onion are peeled, and the monsters are just a required element of the story that the viewer expects. The things you expect to see are there, but only to serve as a backdrop to demonstrate the weirdness and unpredictability of the things you never saw coming.
The more I think about it, the more ingenious I think it is. It's certainly not the best film ever. Maybe not even the best horror film ever. But it definitely was not tired, worn out, stale, or predictable. I had fun with it.
Take all these comments as coming from someone who has always adored the genre.