All the "plain text" formatting languages tend to break down. They work until they don't and then somebody invents yet another text-based pseudo-markup language. Asciidoc is best of the worst but fundamentally it's a bad idea. If you're willing to make the investment in tools and training there's a lot of value in enabling people to write real HTML (and MathML) or, better, Docbook + domain-relevant XML that produces real UI-agnostic structured information.

The problem here is that the tools are terrible or expensive or bloated. There's no reason why this should be the case and it's likely a real market opportunity.

The spec for Markdown's syntax includes HTML in its entirety, though...