A couple of things.
1. If you enable this and find a post that doesn't render correctly then **please file a bug that includes a link to the post and a screenshot**.
2. There's no UI for adding Markdown to the posts you create yet. That's coming, maybe in the next release, maybe not.
3. This only acts on the contents of posts. Content warnings, image captions, and account details are not converted. Not sure if they should be or not yet.