Matrix is the most mature and feature complete. I personally use it day to day through Element. However, if it comes down to talking about something that could

Matrix is the most mature and feature complete. I personally use it day to day through Element. However, if it comes down to talking about something that could get me into massive trouble, I won't trust any single entity. I'd probably use XMPP, or Onionshare in Tails OS, preferably not even on my own PC. The bottom line is that a computer connected to the internet is not totally secure and anonymous, there can be backdoors in every nook and cranny. For example if you are using Element + Matrix on Windows 11/Android, to spy on you, they don't need to have backdoors in Matrix itself. A simple automatic screenshot will do! The OS will take care of that :D But I don't think the Matrix organization can be pressured into adding backdoors into their systems. Our best bet is to use free and open source software as much as possible.