I dont mind discourse for a forum, although it's "one long list to rule them all" is frankly annoying at times and reminds me too much of the modern social media UX where you have to scroll and scroll to find something you care about.
Sometimes, someone only cares about one subtopic.  Yes I know this can be somewhat addressed with proper tagging, but that takes constant effort by everyone involved to make that useful.  Most users wont use them, so its up to mods or other site users to constantly be back filling that information.  It's a constant effort that must always be made to keep things orderly.  It exchanges immediate convenience for recent information for a more long term effort to keep older information as easily accessible as it would have been in a mailing list or a classic forum/sub-forum style structure.  

And speaking of older information... there's another problem with them in that aspect... the ability to work with them offline or local copy is not possible with a SAAS solution like discourse. 

 
As I do a lot of historical research in open source and actively archiving what I can for future people.  This is something I focus on and while I know there's not many of us that are doing it, it's still a thing for some of us.  Working with old Distros and trying to research how we got from there to here has only been possible because people back in the day archived mailing lists and things like sunsite.unc.edu  I can scrape a modern mailing lists for reference later, and pull up mailing lists that others have archived before me.