Slackware vs Nix os

Uses stable, plain-vanilla packages from upstream.

Follows the original Linux roots. It still sees Linux as a free clone of UNIX so the distribution tries to be UNIX-like.

Uses standard tar archives with shell scripting as packages.

Some users don't like to install systemd based distros because they increase booting and processing speed.

Besides Debian (which was first released in August 1993), it is the oldest still maintained Linux Distribution and was first released in July 1993.

It has all of hackage in it's package manager (which is confusingly named "nix" as well) due to being based around hashing and allowing you to compile in a sort of virtual machine (really just changing the PATH variable temporarily) it solves many of the versioning problems that you commonly have with cabal. Here's a tutorial (there's many others as well) http://www.cse.chalmers.se/~bernardy/nix.html.
Also I should note that you can use the package manager a la carte on Mac and most any linux distro.

Since NixOS stores all its packages in isolation from each other in /nix/store and because of the declarative configuration model, upgrading NoxOS systems is extremely reliable. Furthermore, it gives you the ability to roll back upgrades.

Every package in your system is generated from a configuration file. This makes it very easy to reproduce that environment. Just copy the config file into a new machine and it's done.

You can use and test snapshots without rebooting. Booting into snapshots or test configurations is possible without risking the system's stability.

As everything is isolated, you can have on the same machine multiple configurations to meet project requirements that would be mutually exclusive on other OSes.