Asciidoc just seems to be much higher quality in general.
There's a spec being drafted, so hopefully that will spur some more action.
For my purposes, Asciidoc is the best markup with the worst tooling, and Markdown is the worst markup with the best tooling.
:table-caption!: :toc: macro :sectlinks:
toc::[]
That's a mountain of work though and even if there is a spec, at this point it's complex enough and the Ruby implementation is enough of a de facto spec that I'm doubtful you'll ever make it past "this behaves differently to Asciidoctor" territory.
Btw a more recent alternative is https://github.com/jgm/djot which actually does have multiple implementations and looks like a way better option than Markdown, but maybe not as powerful as Asciidoctor still. I haven't tried it.