We've built Relay chat (http://relay-chat.com)! And I believe it addresses some of these concerns.

Basically, it is hosted mattermost. We've written our story of how we created Relay by escaping slack: https://medium.com/@deobald/we-created-relay-by-escaping-sla....

There are bux fixes, and feature improvements pouring into mattermost from around the world, and we get them in. We're building integrations, bridging services and other slack features (which will also be open source) into it on high priority.

There's https://github.com/42wim/matterbridge, which provides bridges from mattermost/relay to practically all other messaging platforms out there including IRC, Telegram, Gitter, etc. We're building a SaSS for this too.

Disclaimer: I'm a part of the Relay team.