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.
I'm using KeenWrite to write a sci-fi novel and can produce a standard manuscript format in seconds (because the content is kept separate from the presentation).
Regarding technical documentation, I extended Markdown in KeenWrite with a consistent cross-reference syntax for tables, figures, equations, etc.
about keenwrite, touting "producing beautifully typeset PDF files" and not giving a single PDF example, ideally its documentation, is a missed opportunity, to say the least ;-)
Most novels don't need anything more complex than a link to the next chapter at the end of the page. Maybe bold text, but a lot get by without. As such markdown will work for most. I've seen some novels with some really complex formatting, but those are rare (I read a lot of litRPG and still call them rare). However I mostly write technical documentation and I often hit limits of things markdown cannot do even though my documents are much shorter than a novel.
Pandoc is a great thing! However, lets not mix up Pandoc Markdown with standard Markdown. Pandoc Markdown intentionally adds features, that make it more suitable for academic writing, I believe.
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