I just migrated my personal website to nixos and can second all of this. There's a learning curve, but the time to provision a new server once it's all working is hilariously short.
I use debian + ansible and it requires discipline (you have to make sure you never do manual steps basically) but my entire ansible playbook makes server creation a 3 min process.
I'm sure Nix is better, I just haven't needed it yet.
> it requires discipline (you have to make sure you never do manual steps basically)
Since Nix requires a declarative configuration, you need less discipline, but more up-front specification. For example, making truly idempotent Ansible scripts requires a lot of effort and some strong assumptions about your starting state and what processes piped changes into your state, and what your state changes really mean. Also, running your playbook with newer version of the same software may lead to a different result. For example, migrating from bullseye to bookworm with a cargo-deb that contained dependencies: It turned out that there were implied dependencies taken for granted in bullseye that were removed in bookworm. With Nix this will lead to a build error rather than a deployment error or a runtime error (in most cases).
Nix requires fewer assumptions.
> my entire ansible playbook makes server creation a 3 min process
I'm a big fan of Ansible, and everything has its use.
I like to categorize deployment tools as either "bottom-up" or "top-down" depending on what assumptions you make about the world: Ansible fills the slot where you have no control of how the server got there, but you gotta make use of what you have, and start from scratch. Terraform is the canonical bottom-down tool: You assume you have perfect control of what gets provisioned, and that it won't go away or go out-of-sync without active maintenance.
In this top-down/bottom-up topology, Nix can fill the whole spectrum; most people assume Nix/NixOS is available to them, at which point their automation starts. Others deploy NixOS via various automated processes that can be integrated with both top-down or bottom-up solutions, e.g. distribute via network boot, VM image repository, or via "hostile takeover" (deploy on existing Linux machines via SSH, like Ansible, or using Ansible).
