I just used a fairly complex GH issue (that I had written on GH) to test POSSE-ing to a test repo:
https://www.svenknebel.de/posts/2018/11/8/ to sknebel/random-test-repo#1

The HTML in my post is a cleaned up version of Githubs HTML (with mentions of users and other issues removed to cut down the noise)

The following things in the created issue were unexpected:

• the nested lists didn't convert correctly
• < > were added around bare links
• in the "output format" section, a space was added before the italicized "not")
• the JSON code example was cut, but I had forgotten to escape a < - the browser still displayed the code following, probably because <------ clearly wasn't valid HTML tag, but it is understandable bridgy (or maybe even mf2py?) failed there (I have since edited the post to use a <)

EDIT: feel free to move this issue to granary or ask me to split it up or ... - happy to help you as much as I can.