Open, and checkable
An encrypted service you cannot audit asks you to take its word for it. The code is open precisely to avoid that.
AGPL-3.0 licence
Both server and client code are published under AGPL-3.0. You may read it, modify it and host it yourself; any modified version you make available must come under the same terms.
What the server sees
It sees who talks to whom and when — metadata it needs in order to deliver. It sees group membership. It sees neither message content, nor attachments, nor private keys: only opaque blobs it has no way to decrypt.
What it does not do
No advertising, no data resale, no third-party trackers. Call media does not even pass through it: it goes straight from one browser to the other.
Hosting
Servers are in France, within the European Union. And if that is not enough for you, self-hosting is documented — the strongest guarantee a service can offer.