My experience is that Asciidoc very quickly gets into advanced syntax.

If you introduce something with advanced functionality onto a team, soon enough someone's going to use pieces that others don't fully understand later.

Now everyone touching the document has to understand includes and document metadata and whatever else someone added. Suddenly you have includes and cross-references and what used to be a document where you could read the raw non-rendered form linearly top to bottom is a confusing mess unless you render it. Markdown is almost always just as legible raw as rendered.