The Signal client may be open source but it's a walled garden. You cannot self-host (if you run a Signal server, you create your own Signal network that has to exchange keys with other networks) and join Signal. Therefore it's a step backwards in our opinion. It's better than WhatsApp, but only slightly. Use Matrix instead.

More details are here: https://github.com/privacytools/privacytools.io/issues/779