That markdown mirror is a pretty neat feature. I've been using Trillium for my notes and the way they save/store the notes is actually one of the things I dislike about it even though I love the application itself. I tried at one point exporting everything to markdown and it worked... but Trillium allows you to have notes in multiple places but they exist essentially as just a pointer in the backend to that note. So it made the export a bit wonky as some instances of a note are just an empty shell and don't have the actual contents. So you have to try and move notes around to get their markdown files in the right places.

I've ended up still sticking with Trilium however as I like being able to have notes in multiple locations like this.