I wholeheartedly agree, and I am a giant NixOS fan, BUT:
1. You need space for the Nix store, so it's not appropriate for every system (I like building small embedded appliances).
2. A six month release cycle can get a bit tiresome after a while. I know it's personal preference, but I wish it was a bit longer.
3. Or, in order to contribute to the project, you need to chase the dragon with the unstable channel.
4. Forcing all the system state through the funnel of a single config file is great, except that it doesn't cover home directories. I found that I ended up with a massive unmanaged blob of state in my home directory which I couldn't capture (I never tried out home-manager, maybe I should have).
This is not to detract from NixOS! NixOS is great, and I remain a massive fan :-). But on my personal systems I find myself using macOS and Debian these days. ¯\_(ツ)_/¯