Morgan Leigh Davies’s review published on Letterboxd:
This one is a puzzle. I read the poem in college and didn't remember it super well but I remembered it enough to sense that certain things had been changed substantially while watching, and afterward looked up the plot summary on Wikipedia and was bothered by just how much had been changed, which feels like a strange complaint about an adaptation of a medieval poem—like, yeah, stuff is going to be different, in theory that is fine. It's been a while since this was written! It's not a sacred text. But I'm not sure why Lowery made this movie! In some ways he's very faithful to the ethos of these medieval romances, to the degree that I think the average viewer would find this movie completely incomprehensible, but in other ways he fails to grasp their inner logic, which is very different from our modern sensibilities.
Gawain, for instance, is meant to be the purest of Arthur's knights, hence his ultimate quest for the grail, and this poem largely revolves around his virginity/refusal of sex when it is offered to him by the couple in the castle at the end of the tale. In the film, Gawain is decidedly not chaste, which is certainly more realistic—people in medieval England obviously had sex!—but I think this change reflects the weird middle ground this film occupies: it's not a complete revision of the story to be "grounded"—it's still very strange and working in an essentially symbolic register—but it doesn't attempt to exist fully in the world of medieval romance. I think that world could work in a movie, you just need to make sure your audience understands that the rules are different. Maybe that film doesn't work for everybody, but this movie is way too fucking weird to work for everybody, so... whatever!
That's just one example of the main problem with the film, which to me is this weird tension between the medieval and the modern, which I don't think is productive but instead just feels incoherent. The whole logic of the Green Knight story depends upon certain notions of honor and duty that we just don't have now and I don't feel that the movie conveys those things effectively. Gawain is a tricky character regardless because he's meant to just be like, this pure idealized figure, but in this film he felt really impenetrable to me. I do think Dev Patel does a really good job with very little material to work with—if he were a worse actor the movie would be really dire because we just wouldn't care at all.
Most of the other performers do very well in their small roles, too—I particularly liked Barry Keoghan and Joel Edgerton—though Alicia Vikander felt miscast to me. Stunning craft work across the board, too—cinematography, music, costumes, sets. But it all adds up to not much. I do respect the ambition and A24's willingness to release such an utterly strange and uncommercial movie (wide release no less! though I suspect that was a calculation based on the expectation that word of mouth would be brutal; I would describe the vibe in my theater as the credits as "nonplussed" and the people seated next to us left halfway through) but I found this very frustrating.