The issue with rclone compared to rsync for existing files is it will copy the whole thing again, where rsync uses something called delta transfer and only transfer the change. But if you are doing one time cloud to cloud transfer then yeah rclone is the gold standard.

> rsync under the hood

Is it? I think one is written in C (rsync) and the other is in Go lang (rclone)

I had the same reaction. rclone is not built on rsync. It's an independent implementation in Go, inspired by rsync.