A lot of odd Nix hate in here, or at least extreme dislike.
NixOS is so very usable. There are a lot of foot-guns but that's true of Linux systems.
It takes some time to get comfortable with all the different ways to configure stuff. There's a new language, a new build system, a new Linux distribution.
I've spent very little time relative to say, figuring out dpkg or systemd, or learning Bash or C or Perl, or learning the Linux FHS.
NixOS makes running deterministic systems driven from a single configuration incredibly easy.
Nix the language and tools allow you to build software deterministically and create environments where that software is available (e.g. on $PATH).
Nix Flakes is an experimental feature aimed at bringing reproducible and hermetic configurations of dependencies into the fold (i.e. lock files, ala Gem or Cargo or Go Modules).
Nix-the-tool's interactive "package manager" should be deprecated, as it's a huge pitfall for most developers. But it works and is frankly a good first step for folks coming from Apt or Homebrew.
Learning to write modules is probably the steepest learning curve, but compared to learning how to build RPMs or Dpkg or Brew recipes, it's really not bad at all. I actually find it easier and I've been at this for less than a year. Sure, if you're trying to get something working no one has figured out yet you may be in for a pickle, but that feels like it's always been true. The difference is a lot of software breaks and bends the rules and boundaries constructed to keep things and people sane. Nix does not allow that, generally, which is a very good thing. Fixing other people's garbage is the bane of any package system maintainer, it's no different here.