I think demon-possession stuff can be scary, or at least upsetting. Not so much when horror comes from a possessed person like in The Exorcist, but more like when demons are toying with the minds of mortals, making them unable to trust their own senses. There's a pretty-good movie no one saw called The Dark and the Wicked (not the greatest title imo) that does this pretty well. Is this person you just met real or was it actually the demon? Was it really them when you first met, and the demon the second time you met? Was it them or was it the demon when you spoke to them on the phone? Or did the demon make you think you were speaking to that person while you were actually speaking to someone else entirely? Were the words coming out of your mouth the same words you heard yourself say? Were you even speaking to anyone at all? It works because the horror comes less from a spooky monster who will kill you, and more from an inability to distinguish reality from illusion.

In a similar vein, one thing I liked about Hereditary was how the demonic forces were establishing "rules" for the protagonist just to fuck with her. I love the bit when she tries to burn the journal and is set on fire herself. So later on when she tries to get her husband to burn the journal, expecting to burn to death along with it, you're like "it's going to turn out that whoever burns the journal also burns, so the husband is going to die." But when he won't do it she throws the journal in the fire in frustration and the husband goes up in flames anyway. There are no rules to this. There were never any rules. The demon was just playing with its food.

So yeah, it's my opinion that if satanic stuff can tap into fears about losing control and losing your mind, it can be pretty scary.