The TL:DR Version: It's another jump scare factory from the Conjuring series, so it's good in that carnival ride kind of way. The thing that sets this movie apart is the incredible cinematography and set design, which makes this film into a true gothic nightmare.
When the boss told me that "The Nun is scarier than Hereditary" I was like naaaaaah. Careful readers will remember just how much I loved Hereditary for the quality of the drama, the attention to occult details, and the breathtaking performances. I'm in no particular rush to see Hereditary again because it's a relentless experience.
The Nun is not as ambitious or as well-crafted as Hereditary. But it's a hell of a lot of fun and I can't wait to watch it again. I'm also pretty sure Guillermo Del Toro loved it.
Like Del Toro, I'm a big fan of stories set in the classic gothic mold; old castles filled with dread secrets, a land corrupted by misfortune, ghosts flitting in the distance, a sinister warning not to proceed further ("God ends here" is such a great warning,) and a wan heroine in white holding a lantern while running through the dark night. It's so on the nose that I'm probably going to own the movie, keep it on mute in the background while I'm working and play some suitably dark music to accompany it.
The plot is pretty standard Conjuring stuff. A nun commits suicide in a spooky old convent in Romania and the Catholic Church sends Father Masculine to investigate whether or not the ground is still "sacred," because giving a clear understanding of what they want him to accomplish is apparently a bridge too far. Father Masculine picks up Wan Initiate Nun, who gets visions of horror as part of The Conjuring Universe's commitment to the idea that God loves to direct his champions via terrifying high speed montage sequences. They're later joined by Frenchy #YesAllWomen, who cannot stop hitting on a novitiate nun despite her giving nothing other than polite deference in response.
The story goes pretty much where you will expect. Creepy things that are set up early pay off later (I really like the bells sequence) and the Nun creeps around startling people and then disappearing. It's total nerdy quibble, but I never quite know what evil wants in these Catholic horror films. We know that Valak getting out would be a bad thing, but it's never really clear why. She does get out, but then all she seems to do is bother families with sensitive children. If that's Valak's thing, hey, good on her. Seems a bit unambitious to me, though.
Anyway, nerd pedantry aside, the movie is my favorite of the Conjuring series. It wins for the sheer lushness of the atmosphere. It's a darkly beautiful film that conjures up the origins of the horror film. It looks very much like a Dracula movie from the classic era of cinema. It's takes what could have been a very paint-by-the-numbers movie and adds an elegance to it.
It's not really the kind of horror that sticks with you. Like all the other Conjuring movies it's just a series of effectively staged jump scares and a depressingly happy ending. But I had a lot of fun watching it and I can't wait to see it again.