Anecdotally, Discord is down and unresponsive far more than Slack ever has been. I would never run Discord as the main method of communication at a large company. It might be fine for when I game, but it's unreliable for anything serious.
Here's some data to back it up:
Slack's uptime is 99.95% from August to October[1]
Discord's uptime is 99.85% from August to October[2]
Here's some data to back it up:
Slack's uptime is 99.95% from August to October[1]
Discord's uptime is 99.85% from August to October[2]
[1] https://status.slack.com/calendar [2] https://discordstatus.com/uptime?page=1