Moving our forums out to a sub domain caused us major ranking issues, and resulted in a dramatic loss of organic search traffic. I knew about the whole seo subdomain vs subfolder issue before we made the move, but thought it was a bit exaggerated. We were desperate to replace our old forum software as it had major issues (zero ability to deal with spammers, mangling content etc) so we took a punt - sadly my worst nightmares were realized :
The above graph is from google analytics, looking at organic traffic to the forums - we switch to discourse on sept 11th. I gave it a while to see if it would recover on it’s own, and spent a lot of time making sure GA etc was setup correctly (it is).