Flathub aims to be the place to get and distribute apps for Linux. It is powered by Flatpak which allows Flathub apps to run on almost any Linux distribution.
How does it stack up against traditional package management and others like AUR and Nix?
The Snap Store is run and controlled by Canonical and is not open source. The rest of Snap is open source, meaning the daemon and core software. [emphasis mine] How threatening this is depends on you POV and has been the subject of much discussion.
if the only way to use the open source client, is with a closed source server, is it really open source at all? The platform is the server.
Exactly, proprietary.