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 sandbox can be very cumbersome when there is not a way to break out. I'm thinking specifically of command line tools for developers. You can poke holes in the sandbox to access the filesystem, but the moment you want to run an executable it won't let you.
Other way around, accessing command line tools. As far as I know, there is no sandbox setting to allow access to execute commands directly on the host system.