At least an originally designed markdown falls back to HTML when it doesn’t directly support a particular feature. With Pandoc, markdown can fall back to LaTeX as well.
You can do the same in Asciidoc with the passthrough feature, either as an inline passthrough or a passthrough block. Granted, it's not syntax-less like Markdown, but it's there if you need it.
Inline example:
pass:[<u>underlined</u>]
or
+++<u>underlined</u>+++
