From spring and summer 2016:
Certainly awful compared to Discourse 
IMHO.
+1
Are there other, more recent and/or substantial case studies?
Constructive criticisms?
Food for thought
Not all related to Discourse …
… we now have a Discourse instance online for you to find help installing/configuring mailcow!
We can finally stop using Github as our forum and keep using it for real issues and bugs.
https://github.com/KeenSoftwareHouse/SpaceEngineers/issues/9#issuecomment-102553658:
Github issues are not made for this type of discussion, you should only use github for discussion on code and implementation or problems, the forums should be used otherwise
https://github.com/WhisperSystems/Signal-Desktop/issues/871#issuecomment-271008417:
… we try not to use GitHub for discussion. I think we have all the information we need in this issue, if you’d like to continue discussion please see the online forums. Thanks!
https://github.com/bitcoinclassic/bitcoinclassic/issues/27#issuecomment-240372693:
GitHub is not a discussion forum, it literally is one of the worst platforms you can choose for that. …
T11268 integration with discourse forums – Phabricator (closed, wontfix):
From a usability perspective, I find the forum software discourse a lot better for discussions …
Why you must not ask questions on Github issues – Medium (2016-08-09)
Two-way sync GitHub issue tracker - As seen on issuepress.co
Last but not least (this topic should not be an exercise in bashing GitHub): Suggestion: Introduce a Discussion tab · Issue #44 · dear-github/dear-github