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]
Perhaps there is some regionality to the issues? I live in Brazil, and Slack is often somehow unavailable: images not loading, message delays, failure to load threads, and the built-in bot commands (not third parties) not responding. And mainly, voice calls having all sorts of connection issues imaginable.
In the last 3 months our team has had to (against company policy) fall back to Discord to be able to do our daily meetings about 6 or 7 times.
I wouldn't run Discord on a large company either, but not for stability's sake, but because their (free) screen sharing is possibly the worst if you want to share a desktop rather than a game (it is very much optimized for low delay, and the downscaling makes text unreadable).
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