I re-watched the Exorcist recently. I found it scary, but in a different way from how people try to sell it.

The head turning, the vomit, the spider walking, that was all shocking (and cool) but not scary.

What I did find scary was all the science/medicine stuff. Doctors telling the mother that Reagan is faking it, or keeping her under sedation, or subjecting her to endless invasive tests. The real horror is the mother's realization that the material world cannot save her daughter. The horror of putting Reagan's life in the hands of faith.

So I'd say that it works on several levels, where for some people, scary is "OMG, a monster face!" and for others scary is the incomprehensible.