I have used markdown to write books, and it worked just fine. Now, these were novels, not technical documents, but I never encountered something that couldn't be solved with markdown plus some occasional HTML.

I've also generated quite a bit of technical documentation using markdown, and Pandoc's extensions[1] allow you to include pretty much any formatting you might require, complicated math and syntax highlighted code blocks included.

That markdown can then be transformed into HTML, a Word doc, a ePub, a PDF ...

There needs to be a very compelling reason for me to reach for something other than markdown.

[1] https://pandoc.org/MANUAL.html