alt Hacker NewsI can see one benefit to a structured yaml for specs like the OP is doing: it gives you more control over what you include in the context window. But coming up with a good schema that doesn't handicap you or add cognitive burden, compared to the freeform flexibility of md/txt, is a challenge.
If the selling point is a new file format for spec management, it would be more interesting to provide an offering with org-mode. The author admits they were unaware of other pre-existing solutions before this project so I am providing context to their critique of OpenSpec.