The reason why it is not easy to downgrade using the Dashboard is due to WD putting code in that checks and blocks prior firmware versions during the upgrade process. See the following thread that explains how to edit (via SSH) the file /etc/version and change the firmware version to allow for downgrading to a prior version.
https://community.wd.com/t/easy-method-to-downgrade-to-previous-version-of-the-firmware/93489
