To me, the ability to chat directly with members of the community and get questions answered quickly is an advantage of a sync chat. Discussions are best held at a forum / community meeting (as long as the meeting is recorded and made available).
Slack is more inviting and better for making connections - I am here adding a comment in a forum which is akin to getting into the center of the market and telling all what I think about this subject. Many members of the community don’t feel comfortable (sometimes on a grammatical level) to communicate a message to everyone (like stage fright i guess?).
To top it off, in this forum there are (I hope) many readers who remain completely anonymous while at slack you are listed as a member and can be contacted by other members - therefore you are a more active member:)
I think both this forum and a sync chat are important and both should exist for a healthy discourse.