[solicode]

The fact that Slack has made it clear they don't intend to support large open source projects is the biggest thing in my opinion. Reactiflux was basically forced off Slack for getting too big. Other large communities like the Clojurians Slack community will likely be forced off soon in the near future too (some members of the community have begun discussing this and evaluating alternatives). The Elixir Slack community is also similar in size, and I see the same thing happening there eventually.