We cover 70+ resources across AWS and GCP (the tool you mentioned only covers 12 AWS resources) and we support a number of integrations including GitHub Actions and Atlantis (https://www.infracost.io/docs/integrations/) so you can see diffs in pull requests. We're seeing a lot of users who are using multiple tools for deploying their services, for example Terraform + Kubernetes or CloudFormation + Serverless. In the future we want to work across multiple tools so we can support these use-cases as well.
Agree, state files can contain sensitive information. We parse the Terraform client side and only send the details to the API that are needed to determine the price, i.e. the instance type, region, etc.