The high sanity of OurBigBook, also makes creating new macro extensions extremely easy and intuitive.
All built-in language features use the exact same API as new extensions, which ensures that the extension API is sane forever.
Markdown is clearly missing many key features such as block attributes and cross references, and has no standardized extension mechanism.
The "more powerful than Asciidoctor" part is only partially true, since Asciidoctor is very featureful can do basically anything through extensions.
The difference is mostly that OurBigBook is completely and entirely focused on making amazing scientific books, and so will have key features for that application out-of-the box, notably:
and we feel that some of those features have required specialized code that could not be easily implemented as a standalone macro.
Another advantage over Asciidoctor is that the reference implementation of OurBigBook is in JavaScript, and can therefore be used on browser live preview out of the box. Asciidoctor does Transpile to JS with Opal, but who wants to deal with that layer of complexity?