Aside from features: I have seen enough corporate mail servers going down, irc and jabber servers breaking etc. Using those won't magically give you 24/7 with

Aside from features: I have seen enough corporate mail servers going down, irc and jabber servers breaking etc. Using those won't magically give you 24/7 with more than five nines uptime. Key difference is: If slack goes down many companies are affected and it goes on the internet. If a company's mail setup goes down employees are a bit annoyed, but after a while it works again and nobody outside notices and it's no news on HN (unless it's a real major thing bringing a notable company completely down)