Any method you forget to overwrite from the embed struct gives a false "impression" you can call any method from mockS3. Most of time code inside test will be:

    // embedded S3Client not properly initialized
    mock := mockS3{}
    // somewhere inside the business logic
    s3.UploadReport(...) // surprise

Go is flexible, you can define a complete interface at producer and consumers still can use their own interface only with required methods if they want.