I guess, for me, its an issue of expectations.
spoiler
The movie didnt seem to be avant garde until that very end where they hit you with (I guess) an entirely metaphorical ending scene. Nowhere else was the film as symbolic or whatever, so me as the viewer is expecting an actual ending scene about the whole life-swapping/pregnant wife situation but instead I get hit with a giant tarantula and have to basically go back to re-examine the entirety of the film for its spider symbolism.
If that intent was built up more throughout the film I may have been primed more for it and could interpret it more openly, but it felt like a total shift in context that essentially made me feel like I wasted all that time.
I put this up there with endings like ‘it was all a dream after all’.
I agree. Hereditary is the best horror movie of all time, so far, for me.