I used S3 a few years ago so this might not be as big of a problem now. But when I started using presigned POST requests to upload documents directly to S3 from the client, there were sporadic problems from the US military, some corporations, and many K-12 schools. S3 would be blocked on their end (likely because someone downloaded a malicious file from S3, and a system admin blocked the whole domain without realizing what they were doing).