One thing I dislike about the use of Discourse for real-time ad-hoc discussions is not having a mobile equivalent or a desktop app equivalent(not that I know of), unlike Slack. The usual flow is: I get an email if someone sends a private message to me on discourse. And often times, I am checking this email on my phone. But I hate to open a browser and respond on my cell phone. So I postpone the response to another time when I am before a computer which kind of increases the latency. On the desktop, enabling notifications is one possible solution but it is inflexible compared to the fine-grained control which slack provides like muting one group, exiting a particular group and so on. Essentially, if I need to have a synchronous communication, I would just do hangouts/slack with the group. For all other more structured long form one way communication, I would use discourse.
Having said that, I think for the fast.ai community, forums alone is better. There is already so much content to catch up on the forums, having a real-time stream can be overwhelming for most people. Recommending only forums makes sure all important content is communicated through forums only. We can always have one off slack channels/groups for interest groups/study buddies and so on based on personal preference. Just my two cents 