IME, not if you're using Terraform with proper modularization and can stick to the commonly available feature set. Have one layer of modules that describe what you want (e.g. a virtual server, database, monitoring) and behind that a separate layer that implements it (e.g. creates an EC2 instance, an RDS database or a CloudWatch alert).

Terraform is not “cloud agnostic” anymore than using Python with the various cloud providers SDKs.

Each provider within TF is tied tightly with the cloud provider.

Standard Disclaimer: yes I work at AWS and I do IAC with the CloudFormation or TF depends on what the customer wants

That's a nonstarter just because of how you describe the various services at each cloud provider. For instance, Google cloud is big into using "projects" as organizers for resources, where the closest analogue in AWS land is an entirely separate account.