There is no AnsibleOS, so you'll not have a reproducible OS, and as the base OS gets upgraded underneath, ansible scripts will break. Also, you have to put a ton of effort into making sure Ansible scripts are idempotent, and even then there are usually leaks. Nix (with flakes) is 100% reproducible and it's very difficult to do anything that isn't idempotent. What this all means is that there is more effort up front learning and setting it up, but a lot less effort and suffering in the long term.

I personally switched for these reasons.