All good and correct advice: Sync IS NOT backup!
Sync can be your first line of defense, if you use a sync service that retains versions. That way, if you accidentally change or delete a file, you can use the sync service’s mechanism for restoring deleted files and/or older versions to recover. Dropbox supports this (I don’t know how well as I don’t use Dropbox) as does SynologyDrive (which I do use). I am not aware the iCloud supports this for files, however, and so I do not see iCloud as providing any type of “backup.”
Sync can also assist in a backup plan. For example, I store virtually any file of importance in one of my folders that are synchronized using SynologyDrive, and as a result there is a copy on my MBAir, Mini, and on the Synology itself. This helps with backup in two ways. First, if I lose my MBAir (or my Mini dies), I can easily replace that computer and connect to SynologyDrive and all my files will sync down from the Synology. Secondly, because everything on my MBAir is also on the Synology and the Mini, I can use a backup strategy on the mini (in my case, ARQ) to back up these files. As a result, any file I create or change on the Air is also quickly replicated to the mini and then backed up by ARQ there, so I only run ARQ on the mini.
Note that I am NOT relying on SynologyDrive as the backup mechanism (other than for the ability to restore deleted or changed versions) but rather as a way of ensuring that everything in in sync and therefore available TO BE BACKED UP by the mini.
(And yes, the Synology also uses its HyperBackup system to send a copy of its data to the cloud as well. I also use SynologyDrive to keep a sync’d copy of anything on the Synology that is NOT part of my active working files on a external drive connected to the mini, so that I have a local up to date copy and a cloud copy, plus the copy on the Synology as well. And I also do a daily clone of the mini (which as my photo library which is NOT sync to the Air) to both the external drive and to the Synology and it is backed up to the cloud via ARQ).