Terraform supports multiple vendors, but your templates are not vendor-neutral, you have to rewrite them all. I don't see it as that big a win over using each vendor's native tools, as long as they all support a config managed in git.
It does plenty of good. I have software that spans AWS, Azure, and GCP (don't ask me why). I can easily manage it as a cohesive whole, e.g. using an ip address provisioned in GCP as the target of a DNS record provisioned in AWS. I can spin it all up in a single call to a single tool.
Under the hood, terraform is just using the AWS APIs. If an error occurs with them, getting support isn't an issue.
