Earlier quoted context omitted.
We are actually trying to make open protocol in https://actor.im with federation with https://matrix.org . It seems that only Actor and Matrix understand importance of federation.
I haven't looked into actor much yet, but the last time I looked at Matrix, my main concern was how much implicit trust users needed to place in homeserver providers (messages and user profiles are stored in plaintext). Unless this has changed, I don't find it viable for any app that values user privacy. Even federated networks eventually gravitate towards a large degree of centralization once the tech becomes mainst…
At least that's how I read Slack's security page (https://slack.com/security-practices) that only talks about "Data Encryption In-Transit" (i.e. server-to-client) and Mattermost's About page (https://about.mattermost.com/) that also talks (only) about encrypted "client-server data transmission".
I do agree that end-to-end encryption is important, but at least Matrix is aware of this and they are working on it (https://matrix.org/jira/browse/SPEC-162).