Oftentimes, resource policies require condition or context keys to be supplied, e.g. https://docs.aws.amazon.com/glue/latest/dg/using-identity-ba...
Or a S3 cross-account replication policy that requires a specific condition to be able to decrypt and re-encrypt the data in the S3 bucket. KMS policies, in general, are notorius for mandating conditions and context keys – at once.
Cross-account IAM resource policies are even more complex, as they require policies to be configured in both accounts, and the policy is different in each of accounts; take Glue as a simpler example: https://docs.aws.amazon.com/glue/latest/dg/cross-account-acc... Such knowledge can't be [easily] derived from parsing and analysing a terraform codebase alone.
Without maintaining a comprehensive repository of AWS services and IAM policy generation rules specific to each of the AWS services that can support a specific permutation of product features used in a project, I fail to see how this product is useful or what it can do beyond trivial «grant the service instance ABC access to the service DEF by using DEF's ARN». Which is… frankly not very useful on its own.