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This is a good way to phrase it. I think I went in expecting it to be scary because people framed it as this crazy horror movie, when it isn't. I still loved it, but wish I hadn't gone in with the wrong expectations
Hereditary is not really a traditional horror movie, which is why it makes a poor comparison. It's a drama movie with horror elements. As a drama, it's incredibly compelling. And the horror elements really stand out when you approach it that way. 
The son accidentally decapitating his sister? The entire family swallowing fault and guilt because no one feels they can assert blame? The overarching heaviness of mental illness being an inherited disease that no one can cope with? That's all what makes it good.