Nix is a bit of a middle ground. Each package has a specific set of dependency version. It calculates the hash of each dependency and compares it to those that you have installed. If it is installed, it uses that, if it isn’t, it installs it. This means that packages can have different versions and dependency hell is impossible, whilst also reusing existing dependencies if they’re the exact same.

I think using AppImage like Flatpak is silly. It is perfect for keeping some programs on a USB drive for example, but not as a way of installed software.