On having your system be managed from configuration its sort of similar to Ansible. One major difference is with NixOS you can easily roll back to previous states of the system. That means both rolling back config changes and also rolling back package versions. That's something that Ansible doesn't really give you. NixOS also forces you to configure things the "right way" (e.g. you can't hand edit files in /etc). That is very good for reproducibility, but sometimes its frustrating when you just want to make things work quickly.
I think the biggest challenge using NixOS vs Ubuntu is if you've got some weird obscure piece of software you need to get working there's a better chance that someone has already figured that out on Ubuntu and you might have to do the work to get it running on NixOS.
On the other hand I've found contributing to Nix easier and less intimidating than contributing to Ubuntu. To add a package to Nix you just open a PR in the nixpkgs repo on github. I've found the community to be friendly and helpful.
I use a lot of LXD containers for when I just want play around with something in a non Nix environment.
Oh and I love being able to run `nix-shell -p <package>` to use a package without "installing" it.
NixOS is leauges above Ansible and similar. They are barely even playing the same game.
The TL;DR is that Ansible is given a description for some part of the system, then squints at that part and trys to make it match the description. This means that it doesn't unify anything that you haven't described and if you stop describing something it doesn't go away (unless you explicitly tell Ansible to remove it). This means that your Ansible configs end up unintentionally depending on the state of the system and the state of your system depends on the Ansible configs you have applied in the past.
NixOS is logically much more like building a fresh VM image every time you apply the configuration. Anything not in the configuration is effectively gone (it is still on the filesystem but the name is a cryptographic hash so no one can use it by accident). This makes the configs way more reproducible. It also means that I can apply a config to any system and end up with a functional replica that has no traces of the previous system. (other than mutable state which Nix doesn't really manage.)
Nix has other advantages such as easy rollbacks (which is just a bit more convenient than checking out an old config and applying it manually) and the ability to have many versions of a library/config/package without conflicts or any special work required if you need that.
I think the biggest challenge using NixOS vs Ubuntu is if you've got some weird obscure piece of software you need to get working there's a better chance that someone has already figured that out on Ubuntu and you might have to do the work to get it running on NixOS.
On the other hand I've found contributing to Nix easier and less intimidating than contributing to Ubuntu. To add a package to Nix you just open a PR in the nixpkgs repo on github. I've found the community to be friendly and helpful.
I use a lot of LXD containers for when I just want play around with something in a non Nix environment.
Oh and I love being able to run `nix-shell -p <package>` to use a package without "installing" it.