In my experience, using & configuring NixOS is much simpler and easier than most distros...until it's not, at which point, yes, the difficulty curve is very steep.
Even in those cases, though, it's got the nice feature that once it's solved, it's solved. All the config is in one place, and you can apply the same config across all your hosts. You don't get those situations where you get something working after tweaking configs, env vars and systems jobs, and can't remember how to reproduce it.
And though some of the conventions around NixOS are pretty arcane and convoluted (though the language itself is pretty simple), it has the advantage that you only need to learn it once and you can use it all over, instead of switching between bash, python, yaml, toml, systemd units and even more obscure formats (nginx, apache, postfix, etc).
IMHO, the tradeoff is completely worth it.
The problem with Nix/NixOS is everything is too isolated to be configurable or usable, and so almost everything is broken because it doesn't work like any other OS. Chasing down all of the oddities and gotcha problems becomes a full-time IT job that slows everything else down, and then with a fragile special snowflake of patches and workarounds, it becomes very expensive and slow to do anything or to count on it being reliable. Furthermore, the syntax and nature of the packaging DSL is also too clever special snowflake that could've made do with declarative data or imperative procedural with something widespread like Python, Bourne shell, or JS to reduce the friction of customization and contribution.
Like Qubes, an over-optimization for particular goal(s) ends up becoming Achille's footguns because of a lack of compatibility and usability with everything else that came before.