Okay. Sorry in advance for the rant.
Great buildup. Eerie atmosphere. An unusual isolated location. A compelling mystery. Imagery that's creepy in an uncanny way. Some interesting ideas. A visual style (and a couple other elements) that reminded me (positively) of A Cure for Wellness.
However, while something like the slasher genre can afford to have a number of unlikable characters due to their large casts and the fact that the majority of them are meant to be unsympathetic in order to grant a sense of catharsis when they're killed horribly, focused thrillers with one lead that you're supposed to be rooting for don't benefit from the same allowance. In Cuckoo, Gretchen has literally nothing to recommend her. Until a point halfway through the movie (which isn't hidden but she treats like any other so you kind of forget about it despite it completely changing her motivations), she's unfailingly moody, rude, incompetent, stupid, ungrateful, selfish, reckless, arrogant, lazy, awkward, aggressive, dishonest, entitled, and insincere, and despite all that, she's indescribably boring, ensuring that one couldn't care less what happens to her because she fails to display even a single positive personality trait. I actually had to triple-check to make sure it wasn't the same writer as Dark, the paragon of creating uninteresting, unlikable characters. Though Schafer's acting doesn't do her any favors either.
Besides that, there's sloppy, repetitive exposition and some glaring scientific/logical problems if you think about the plentiful explanations they provide for more than 5 seconds. I only finished it because Dan Stevens' charm, combined with the aforementioned elements, made it watchable, but jeez it was a slog.
Would not recommend.
