Mattermost is an open source, self-hosted, private cloud messaging solution compared to Slack which does not have the option to self-host. Security concerns still remain the number one barrier to a cloud computing deployment. With Mattermost, compliance is easy as the data does not leave the customer's premises. It’s perfect for businesses that want to keep their messaging app under tight control. Slack offers 2000+ integrations which covers all applications businesses use. While Mattermost has 100s of integrations as well, the use-case coverage is still limited when compared to Slack.
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.
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.