Discord displaced Slack in our organization and we're never looking back at Slack ever again.

In my experience it displaced Mumble and Teamspeak only for casual gaming. If your group is strict enough about muting/kicking people who refuse to fix their mic settings, Mumble or Teamspeak is generally better -- better quality, local volume adjustments, less resource hungry, open mic not being the default, etc. Think tryhard esport players, or just people with old hardware (probably 90% of CSGO players )).

For my group this wasn't about mic settings but mumble being bad out of the box. Maybe because we didn't pay for the servers and used a free something, maybe because of bad luck, I don't know, but every experience we had with mumble was terrible. For teamspeak, while the voice part is great, the message part is (or was at least in ~2016) lacking compared to discord. We were coming from skype/twitter which had better support for """rich messages""".

Yeah, the text chat is just crap. I think most people use a separate thing for that, probably Steam in the PC gaming use case.