The core features are pretty much universal and used in a lot of apps (chats, forums, git repositories, project management). Custom flavors maybe not rendered correctly at times but this doesn't make documents unreadable
The core features used in a lot of apps include different flavors, like GitHub Markdown and others. The closest thing you have to a unified Markdown is CommonMark and even that is forked for GFM. Yes, headers, bold and italics text, and lists are nice but there is a lot more to take into account when you are trying to supplement docs that other extensions add.