Rocket.Chat varies from Slack by not having anything similar to Slack Workspaces. It has channels, private channels, direct messaging and even OTR (Off-the-Record) chats. In an OTR session with another user, all messages will be end-to-end encrypted and will be deleted after the session. Like other open source alternatives to Slack, Rocket.chat also loses out on some customers because of the lack of integrations with the rest of the business ecosystem.
Zulip is a powerful, open source group chat application. Zulip provides the benefits of real-time chat, while also supporting asynchronous communication. Zulip is inspired by email’s threading model: every channel message has a topic, just like every email has a subject.
Element is an open-source instant messaging client. Element supports end-to-end encryption, groups and sharing of files between users. With Element, conversations are hosted and owned entirely by the participating servers, instead of being stored centrally somewhere in the US, which is particularly attractive to European governments and military.