Not sure what the plot holes people were complaining about. I was a bit confused about why the mum was suddenly possessed, but other than that it all made sense. Grandma leaving her demonic plans in a box in the attic was silly though.
The main one for me was that the orchestration of the cult's masterplan was implausibly convoluted. We're to understand that the host's head must be removed in order for the demon to pass from body to body - fair enough, occult shite - but the way they managed to remove the daughter's head was a bit fucking convoluted.
"Right, let's hope her mum uncharacteristically sends her to a house party she really shouldn't be at, hope she eats some of the nuts there what with the allergy and everything, and hope the son then drives recklessly while she sticks her head out the window trying to breathe. We'll chuck a deer carcass in the road to ensure he swerves just at the right point so her head gets knocked off by this telephone pole we've put our sigil on". Either that, or it was just happenstance, which is almost as stupid. The demon itself seems fairly useless if it wasn't able to escape that girl for 13 years without help from a bunch of naked fat people, which makes you question its effectiveness as one of the Kings of Hell.
Also, when the girl's head got knocked off, it went into the book - right? For some reason - okay, occult shite. Then it gets into the mum when she throws the book into the fire (for some reason setting the father on fire in the process, even when that wasn't what was set up earlier when the mum tried to burn it). Okay. Then the mum has to cut her head off to let it out so it can transfer into Peter, who just so happened to have been so spooked that he jumped out a window to his death. Again, maybe happenstance, but then the movie ends with a ceremony immediately after where they basically explain that this was all planned (thus fulfilling the labored dollhouse metaphor). Just seems there would be a far easier way to get the demon into Peter than all that. Also, why are they explaining to the demon that he is, in fact, a bloody powerful demon at the end? Again, this demon seems pretty useless and not worth all the bother.
Fair enough, there's not always much logic to the supernatural, and it's definitely not a genre which benefits from overexplanation. The problem with Hereditary, as opposed to a genuinely ambiguous supernatural film like The Shining or a supernatural film with a clear story and vision like The Exorcist, was that it tried to explain everything - complete with a purely expository monologue at the end - but the explanation made no sense. It really feels like a first draft by a writer whose ideas changed several times throughout the process.