I want to chime in with one editor that I think nailed a markdown WYSIWYG: https://www.typora.io/

It's hard to explain the concept with just words, but I will try: While you're typing, markdown is fully exposed, but when you move the cursor away, the formatting characters disappear and you just see pretty formatted text. As soon as you move the cursor back to the formatted text, the markdown reappears so you can edit the plain markdown syntax.

I've been using this editor for two years and I think the developers were really clever in this implementation; I've never experienced any unexpected issues. Slack, on the other hand, has been driving me absolutely crazy. (Try changing an inline code block after you've created it. Easy in Typora, a nightmare in slack.)