Slack gets a lot worse once you want to start dealing with two teams using it. With most IRC clients, I can have connections to multiple servers running, and able to see at a glance everywhere that there's activity. With Slack/Discord/etc, that involves a frenzy of clicking as you look through each group of chats individually. I can lay things out how I want, move things around, etc. Slack, fortunately, has bridges built in for IRC and XMPP clients, so I can use my already working clients instead of having to keep another web page open to another javascript-laden nightmare.