One of the huge wins for Markdown is that it exposes the stylistic intent of the author clearly, while only slightly compromising presentation. It's a shame that Slack didn't realise that when they switched to a WYSIWYG editor.
I find it most annoying when editing, and I'm not sure whether I'm inside or outside the marked-up region. The auto insertion of ``` was fine (on both Gitter and Slack) because it was clear enough in both cases that they were there, which made a significant difference.