Sorry to be blunt but I've seen nearly exact comments as yours and they come off as someone who hasn't really used Nix and doesn't understand the value proposition and the problems it solves.
I've used Ansible for years, then moved to Nix and have been on it for a year and a half. Yes the configuration language is more complex than Ansible's but it's also way more consistent and predictable. The bit of complexity comes with a lot of additional benefit. Ansible half-asses it, only giving you idempotency if you stay within certain boundaries in how you write configs. And a leaky system is a system that will break. Making it even worse is that you are building on top of a moving target with a distribution underneath that will break your Ansible scripts regularly as the system changes. Ansible is a huge mess and not suitable for the job IMHO.
With NixOS, the entire system is controlled by config, so there is only one source of truth for the state of the system, unlike Ansible. This is a fundamental flaw. Ansible is a hack on top of pre-existing setups.
With Nix, my system only gets more stable with time. It's the opposite of anything I've ever used, where systems get crufty and have to be reinstalled every couple years. Since the state of the system is fully determined by your config, you can always get to an exact state without a bunch of extra shit missing or added.
And the language isn't really that hard for anyone who's done a bit of programming. It's certainly easier than most general purpose languages, as it's static data 90% of the time with a bit of logic thrown in when needed.