How does terraform compare to ansible?
I find few if any reasons to use Ansible over a shell script. IMHO Ansible is just a weird YAML syntax to generate a "shell" script with some utilities to ship that script to nodes over the network. I find it super awkward not to mention slow and inconsistent.
For deployments I much prefer using Nix and for imperative actions I just use actual shell/python.
The reason to use ansible over a shell script is that the ansible playbook will be idempotent. That is to say you can run/rerun the playbook from any point without having to wipe any previous work, or worry about double applying your config changes.
This isn't really true. I think you are correct that most of the built-in operations are idempotent but you can also do this with a small library of functions in a shell/python script or whatever you prefer. Most things you want to do on provision are idempotent anyways (install this package, download this file) or are trivial to make so (create this directory).
I would take a real programming language any day for the minor cost of having to handle idempotency myself. It would take a couple of hours to reimplement idempotent primitives to replace the Ansible standard library in just about any language.
In my mind the main value of Ansible is playbooks that others have made for you, but many people avoid these anyways to have full control.
I personally find Ansible's value lies in what it makes difficult.
Ansible is a list of actions that you apply linearly. Each action might be a noop if it already exists.
Terraform is a tree of resources that are applied by order of dependency. Terraform also records the previous run and deletes resources that are no longer in the code.
Generally, Ansible is great at performing actions on a lot of hosts. A sort of multi-ssh. And Terraform is best adapted to manage cloud resources.