It is an more private alternative to Discord. Even tough they do not offer federation, self-hosting servers is possible, but self-hosting Dms is not possible. Matrix does not have separated voice and text chats in one server, aka Discord style. Is it better to use Revolt for now or create our own Federated Discord alternative?
Revolt: https://revolt.chat/
This is a voice chat app unlike rocketchat and the other you name.
Voice-chat is only a minor part of Discord, or otherwise I would have suggested Mumble, which as far as voice-chat goes is much superior to Discord.
And I think some of the above include voice and video calls. XMPP certainly does.
Oh, I always saw it as a voice chat client with text chat functionalities, never thought it was the other way around.
Good to know.