I've started to use nix (both nixos and nix installed on ubuntu) to manage those critical packages where I want both control and the option for major updates outside the OS's release cadence.
Is there anything special about getting nix working well with other package managers? I'll be honest, the main thing keeping me from digging into it further are the docs and the syntax. I can never tell if I'm reading about nix-the-package-manager, nix-the-language, or nix-the-operating-system; and looking at the syntax makes me understand what most programmers feel when they look at lisp.
Sorry to hear you had issues on ubuntu. Its hard to say how you might improve your experience without knowing more details. The nix forum is probably a good place to get support for that sort of thing.
It only clicked for me when I started using NixOS (on my primary laptop in my case) rather than just Nix.
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.
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 wrote a blog post a while ago that tries to go a bit more into detail over what I just described https://kevincox.ca/2015/12/13/nixos-managed-system/