Group voice is discord’s biggest feature, but also general messaging (both direct/group as well as communities), video streaming, and file sharing.

Element might be a viable alternative, I’ve used it for chat where it certainly is, I haven’t used voice but I’ve heard some people say that video has a very high delay. My friends and I will often share screens for one reason or another and I’m not sure that the experience would be comparable in Element yet.

Mumble isn’t comparable because it doesn’t implement any of the same features other than voice, it doesn’t have persistant logins, etc.