I never used Haml, but I use Slim (which looks similar) and have lots of frustration with it.
Adding Markdown to this feels like nightmare. I don’t know, maybe Haml is better designed than Slim. I have similar experience with yaml, it’s also indentation-based and has lots of syntax sugar. Had the same problems and frustration with it.
Editing erb templates (with help of editor extensions) feels so relieving after Slim.
I would never mix Slim and Markdown. It would be simply impossible to differentiate one from another. Haml and Markdown have not that many clashes in syntax.