Terraform is great for certain tasks, but even they advise against using it for local execution. Whatever you use it for you really need a provider and their module system isn't very intuitive either. Ansible has/had potential, but sucks in a lot of ways too. Unfortunately, as much as I dislike certain aspects, it really is the best generic automation tool available at the moment.
Yeah, if you need to manage individual servers/VMs, it's not a great fit. I've used cloud-init files to configure EC2 instances on startup with things like packages and SSH keys, and that works pretty well if you can treat those servers as if they're immutable. But if you need to get in there and run something, it's not quite a replacement for Ansible.
Yet people use daily to manage 10k-100k+ servers/devices.
The term "scaling" has very different meanings depending on context and how one product scales is very different from another.
You could setup a context that favors push vs pull and vice versa, you can also see different products scaling well or not depending on slight variations in context and implementation.
I am highly dubious that it would be possible to manage 100k servers, specially to do interactions with large numbers at a time. The way tower collects results in a thread pool, assuming success, simply does not work at any scale. I tried and tried. I fixed many bugs and got to about 4000 hosts before changing to another platform.
If almost every server is reliable, I am sure it would work fine. That is not going to happen at scale.
