How does it stack up against traditional package management and others like AUR and Nix?
I only used AUR for a few packages (<5 at a time). It's to be avoided and only used if the other options are a massive pain (unless it's an official package).
Then I left Arch and eventually landed on MX. During that time Nix with home-manager has slowly replaced flatpak, and I don't even have it installed anymore. Nix is better in every way, except for ease of use.
Flatpak has great gui integration (for gui tools). You can click through everything, and the updates are unified. It usually works perfectly fine if you just need to install a few programs.
With nix, there's a lot more setup, but there are many benefits. You end up with a list of packages, and that's really useful because you can take a fresh install, install nix and home manager, and then run a single line to reinstall everything. You can rollback updates, pin specific versions, install packages from a repo (if it has a flake.nix with outputs), and also configure them. I'm using the unstable branch, and it's giving me bleeding edge packages on Debian. And there's no risk of outdated system libraries, like with flatpak, because it provides everything.