Riak CS been dead for over a decade which makes me question the rest. Some of these also do not have the same behaviors when it comes to paths (MinIO is one of those IIRC).

Also, none of them implement full S3 API and features.

There's a difference between S3 API spec and what Amazon does with S3 - for isntance, the new CAS capabilities with Amazon are not part of the spec.

Ceph certainly implements the full API spec, though it may lag behind some changes. It's mostly a question of engineering time available to the projects to keep up with changes.

What does RadosGW miss?