Day 251 of the challenge! The Nun (jump scare) isn't very (jump scare) good. It's kind of bad (jump scare).
Think back to the most frustrating horror movies you can think of. What frustrated you about it? Illogical characters? Bad decision after bad decision? A reliance on cheap jump scares? You get the point. You could fill a book with everything that can make a horror movie forgettable and uninteresting (pretty sure there are actual books like that), and then use said book to make something like The Nun. To say it's predictable is an understatement. What's worse, it tries too hard to stand out amongst the other offerings in The Conjuring Universe, with the effort making the film feel disconnected from everything else.
One of the biggest appeals (to me) of The Conjuring film series is it's usual "less is more" approach to scares. They present very mundane and real world settings and characters, things you wouldn't take a second look at, and corrupt it as the story and hauntings unfolds. The quality of the individual stories aside, I feel the series as a whole does a great job with this. The Nun does the opposite of that, making the action take place in a decrepit abbey that is so obviously haunted and twisted, it felt like a dark fun-house version of Hogwarts.
Some complain that the cinematic Annabelle isn't as scary as the real life one (I agree), and The Nun doubles down on that. Where can you take the atmosphere and story when it's designed to look so evil from the get go? Into comedic territory apparently, because I found myself laughing quite a bit at how convolutedly outlandish everything became as the film went on. Add in an elaborate and epic feeling backstory (down to the one almighty mcguffin) and you have a film that feels more at home in the similar feeling (but lesser) Insidious Universe.