Ansible looks fine when you start, but gets painful.
You need to write idempotent ansible from the start. No excuses.
Then you would implement a retry logic in ansible since your cloud APIs will fail for weird reasons or you really must wait that a resource in AWS is truly ready before you run another task. ( for example vpn gateway in AWS must be ready and then you can attach a vpn connection, or a route53 zone record has to propagate before you do something with it. But in the meantime your other tasks can continue in parallel. Only the vpn tasks needs to wait)
Then after a while with all your resources created with your idempotency implemented in ansible, your code will roughly first check ( with timeouts, retries) if you actually can skip tasks since there is nothing to do. The checks increase, your playbook takes longer and longer to execute. Maybe you try to parallelize it in some way.
In the meantime a colleague of yours executes the same playbook in their machine or pipelines, and now you see weird side effects. Maybe you start implementing some locking behavior.
At this point we did not do any code reviews yet and we did not answer the question: “how should the infrastructure look like?”
This especially means: how do you actually delete resources with Ansible? So you Start to introduce a kind of “state” for your resource.
So, if you squint hard enough, idempotent, resilient, parallelizable, thread-safe, fast ansible with a state is what terraform solves.
If you look at tools like terratest you even get to do unit/integration testing for your infrastructure code.
So as soon as you are more than 1 person handling infrastructure following best practices like code review and testing, without getting insane, use terraform.
If you are alone and don’t want to follow the above best practices, you still have to be very good in writing idempotent, resilient ansible code.
My hypothesis is that the intersection of people who write high quality (idempotent, resilient) ansible code but people who do not care about code review/testing, is quite small.
Once you do not work in isolation any more, either in a team or as a consultant who needs to hand over their work (and Teach people how idempotent ansible works), I would favor terraform any time.
You could even call ansible from terraform if you need some ansible integration, where ansible has a nicer api for you. But let terraform handle the retries, state management and ansible could just be the executing part. So ansible is more like a fancy bash script/function which you call in an orderly manner.