Markdown isn’t detailed enough for legal stuff. Internal references, tables, complex section numbering require extensive post processing or simply don’t work. You quickly wind up with a lot of hidden magic that frustrates people used to word.
Last time I lost patience with doing Legal stuff in word and evaluated alternatives, I was most optimistic about Asciidoc. Unfortunately the ecosystem was relatively anemic… the strong syntax was limited by the tooling.
Looks like there’s been some improvement, maybe I’ll try again. There’s a nice new homepage at least: https://asciidoc.org/