It's being used a lot for code documentation, but MD is bad fit for documentation. Good documentation requires a lot more formatting and cross-referencing than you'd think. Every language I'm using that has been using MD (for embedded and online docs) added extensions, resulting in more fragmentation.
So in general I switch between 3-4 differently-flavoured markdowns constantly. The extended syntax used by various projects sucks. Where did the simplicity of MD go? It reads poorly when unformatted, which is how I read it 99.9% of the time.
I gravitate more towards RST as it's a bit more widely used, but also agree about AsciiDoc.
Do you have any ideas re link-rot, or underlying assets changing and semantic links becoming invalid? How do you reference a code asset whose name could change, what will not only replace all references in the code but also in the documentation?
But that steers you back towards markdown. How deeply do you not like markdown?