Nix package manager vs Flatpak

Every time your profile changes, you get a new generation of your profile and older generations are kept around, so you can easily (and atomically) revert to older version of your profile.

Since packages are stored in the /nix directory, it won't overlap with anything you'd install in /usr/local or wherever else.

You can have different (probably overlapping) sets of software installed in two or more profiles that will be handled (changed, versioned, upgraded, reverted) independently. All software will be installed in the same /nix/store, so any overlaps between your sets will be physically installed only once.

Every time you install, delete or change anything you get a new fresh copy of your user environment (set of symlinks to files in /nix/store) that's stored in the same /nix/store and handled mostly the same way. Your "profile" (symlink to one of environments) is updated after everything else is ready, so you'll never end up in a half-finished state of your system.

Everything is owned by your user (unless you use more powerful multiuser mode that doesn't require root either).

Due to its functional nature, it can just download a binary package with the same hash if its available and it'll get the very same package as you'd build locally (to the last bit that is).

Nix treats packages like values in a functional language. Since they are built by functions without side effects they never change after they are built.

You can share packages via HTTP or SSH (push or pull) with just some simple commands.

Requires just one shell script to install, only one directory is created via sudo. This makes for a pretty simple install method that most can grasp.

Since it is compatible on Mac and Linux, there are many apps in their repos.

You can install flatpak packages on any distro you want.

searching, installing and updating are faster than others in my experience

Plus it's not proprietary.

All applications are limited to a set of predefined permissions, enhancing privacy and security.

You can install a lot of runtimes for different apps, making applications a lot more compatible while still allowing some applications to share their runtimes.