Nix seems more competitive with something like Conda or Spack than a traditional package manager like Apt or Pacman. Similarly, NixOS seems more suitable for deployment in containerized environments built on woefully outdated deps than personal dev setups with generally higher security requirements.
I've experimented with both Nix and NixOS, and while I adore both the beautifully functional language and the endlessly helpful community, I haven't been able to justify the overhead of maintaining a nix.conf, flakes or no flakes, on top of everything else I need to do on my machines. I kept running into issues where some program I needed wasn't already integrated with the Nix way of doing things or something that worked on my Linux setup failed to work on my macOS setup despite multiplatform guarantees.
Nevertheless, I wait with baited breath for the day that Nix replaces Conda among a critical mass of practitioners in my field. I've tried getting a few of my teams to switch (with help from Devbox and Devenv), but it's a struggle to get people to learn new things when their (often worse) way of doing things works well enough most of the time. Heck, I count myself lucky for getting some people to switch from the atrociously slow Conda to the significantly faster Mamba as even that was inordinately difficult despite for latter being a drop-in replacement for the former.