Short Review: The movie is perplexing and unnerving, I didn't find myself frightened or particularly horrified, but I found myself extremely uncomfortable and unnerved the whole time. Something this studio excels at. If you are experienced in Horror films you should find it as a fresh and novel experience. If you are new to Horror, I'm sure you'll be huddling at many points, totally unsure about what is coming next. I'm not sure the studio will ever put out something quite like Hereditary again, and Saint Maud is certainly no masterpiece, but it deserves to be watched at least once; and given your full attention when doing so.
Long Review: I want to premise this by saying that Horror is not only my favourite genre of movie and game, but also increasingly the focus of my graduate studies in (let's just say) cultural theory.
The movies this studio is putting out play around a lot with uncanny and unsettling cinema in a way that is aesthetically and stylistically pleasing and complex. Though, I can't help but feel like the studio is riding the success of the films Ari Aster wrote and directed (being Hereditary and Midsommar), while this film was written and directed by Rose Glass; considering the fact that the trailer advertises its connection to the previously mentioned Ari Aster films. This is not the studio's debut film, which was actually Under the Skin or Enemy, both made in 2013, it actually has about 25 films under it's belt.
However, this studio's recent films play around with your sense of comfort and consistency by constantly throwing uncanny imaging your way, utilizing time-images well by quickly interrupting long shots, using an interplay of volumes to quickly create and release tension (often to make you put your guard down), and by using genre expectations to build and then misdirect viewer anticipation (which helps in 'the scare'). It is for these reasons that the studio is quickly developing a strong cult following.
However, due to all this it must be said that Saint Maud does not follow the usual path of a Horror film, it operates with much more subtly than, say, the Conjuring franchise that has done so well and has sort of developed a Horror cinematic universe. This is not to say that A24 is the Avant Garde of horror we must follow without question, but that I see a lot of the innovation in A24 movies as in the Blair Witch project (which massively popularized the 'found footage' Horror genre of the 00's-10's).
This also means that the movie isn't for everyone, as the Horror genre has huge separations and distinctions in it. Saint Maud is a deeply psychological experience, inviting the viewer to sort of 'scare themselves' by offering a window into an experience that attempts to 'pull the rug out' from under their perception. Jump scares are rare (if happening at all) and there is no 'big bad dude', but only the fragility of our own minds; which this movie attempts to take full advantage of.
All-in-all, if movies like Silent Hill, The Women in Black, or The Conjuring are your horror preferences, you may find this movie novel and creepy at best, or slow and uneventful at worst. But, if you let yourself get unnerved by it, you'll do what I did after I finished the film, and sit there in astounded silence for a few moments just digesting the novel experience from this great studio.