This is really great. Two big missing features preventing us from switching away from Slack are direct messages and mobile push notifications. The latter is hard to do: push is highly centralized, which is really bad for self-hosted/open source software. But DMs are totally doable.
Why is this the case? I would have thought it's the same in principle whether it's a central Slack server, or a central FooMumble server running Let's Chat.
(Difficulty of implementation might be high, but that doesn't seem to be what you're talking about?)