A major limitation of Markdown is the lack of standardization. For example, even within GFM there's multiple subtle variants: a single new line becomes a space when rendering Markdown files, but a line break in issue comments.
If you allow Markdown input you have to give a cheatsheet showing which "flavor" you are using.
This sucks for sharing documents with other people, but in practice it's not a problem. 99% of my writing never leaves my notes app or blog. And when it does, I often export it to PDF or Word to make it easy for non-techie people to read (I love Pandoc for this, it's easily one of the favorite tools in my daily toolkit).
https://blog.codinghorror.com/standard-markdown-is-now-commo...
So we are left in the mode where markdown remains needlessly fractured. Different flavors that render slightly different and everyone gets a little annoyed but they deal with it ands occasionally bemoan the fact that there’s no standardization.
I suppose, since Commonmark specifically has HTML block and raw HTML inlines, any chunk of HTML is by definition valid Markdown.