I run a Matrix instance on my own hardware for my extended family. I suppose that I could be served with a subpoena/warrant for the data, but the contents of any voice or video calls mediated through my Matrix server wouldn't be preserved.
Likewise, any private chats on the server would remain encrypted and I wouldn't be able to decrypt them even if I wanted to do so.
Since the instance isn't federated, and access is only available through invitation, only those who have access know about.
As such, I'd say that private chats and voice/video calls through my Matrix instance are pretty secure.
I run a Matrix instance on my own hardware for my extended family. I suppose that I could be served with a subpoena/warrant for the data, but the contents of any voice or video calls mediated through my Matrix server wouldn't be preserved.
Likewise, any private chats on the server would remain encrypted and I wouldn't be able to decrypt them even if I wanted to do so.
Since the instance isn't federated, and access is only available through invitation, only those who have access know about.
As such, I'd say that private chats and voice/video calls through my Matrix instance are pretty secure.