The discord interface suffers from modern design syndrome, the information density isn't there. I open it and see inline images/gifs, link previews, reactions etc. There's a bunch of padding and avatars around each message. I know some people consider those things features, but I personally hate them.
Mumble has much higher quality and lower latency than discord, but that's probably a matter of configuration and server location.
No limitations on what you can do with bots either. We do all sorts of stupid shit with irc -> mumble text-to-speech along with our music bot and clip soundboard. I'm guessing it's probably possible to recreate it all on discord, but I can't imagine rewriting it all.
I suspect the Discord API's are significantly more complicated than irc/mumble.