It is worth noting that rclone replaces the transfer utility part of rsync. Not the efficient transfers part (which also isn't compression but may be what you meant)
To be clear, rsync is two things:
Tool to mirror (or clone) a directory structure from one source to another. It is done either locally or via SSH. It is one-way mirror.
Algorithm used by the tool to do these transfers efficiently when a small change is made. With rsync, if there is a small change to even a big file, very little data other than that change needs to be propagated. This actually makes it less efficient for a big change to a big file but the end result is still the same file.
rclone is like the first bullet for rsync. It does directory cloning but supports a ridiculous number of backends including some meta (wrapper) ones for compression, encryption, chunking, etc. But it does not do anything with existing data.
If you just have SSH (or local) remotes, which one is better depends on your use case. rsync, to my knowledge, does one transfer at a time while rclone can do many. But rsync can reuse data better than rclone. So if you have big files with small changes, rsync is better. If you have lots of little files, rclone is better.
Obviously if you want to connect to anything but SSH and local or you want encryption, then rclone is your only option.
rclone also has the ability to mount or serve a remote. This is unrelated to the comparison with rsync but is a common use case.