rclone is a nice cloud cloning solution for files and folders. casync is intended to clone and delta entire filesystems, in a way that makes them nicely deployable. rclone is very cloud focused while casync says nothing of the details of how images are served.

casync also has fs composition, a multitude of recorded file attributes, automatic reflinking/hardlinking, uid/gid shifting, and so much more.

tl;dr: rclone is for files, casync is for entire filesystems/deployments.