It isn't that it's very metaphorical, traditional to the folklore, or just an overall atypical film. It's that this WAS NOT advertised as such. The trailers made advertised a much different movie than what audiences will experience. Since this this is a theatrical release instead of a streaming release it is literally cheating people out of their money.
Personally I feel about this movie the same way I feel about Catz. Just as Catz was great on broadway, and The Green Knight sounds great as a folklore story. However, when translating these mediums to films, especially for the general audience, you have to either adapt the story instead of word-for-word, or give enough explanation (whether spoken or visually) for thise unfamiliar with the IP to keep track of during the movie.