Depends on the use case. For one on one messaging or small group chats, Signal is great! If you need a Slack/Discord replacement, Matrix is your go to.
Is Nix good? Afaik the main selling point is that the package manager stores different software versions by hash, so all are accessible. Not sure how that works with everything else
I'd be fine with using it as my messenger of choice if they open sourced it and allowed us to run our own servers