The S3 API doesn't have the concurrency control primitives necessary to guarantee consistency in the face of concurrent writes.

I wrote a distributed lock on Google Cloud Storage. https://www.joyfulbikeshedding.com/blog/2021-05-19-robust-di... During my research it was quickly evident that GCS has more concurrency control primitives than S3. Heck S3 didn't even guarantee strong read-after-write until recently.