Four issues with using sync drives for backup

Limited space. Compared to most backup services, sync drives tend to offer much, much less storage capacity. Many backup services are unlimited, or offer much greater capacity than a typical sync drive.

Limited versioning. Most sync services do now offer some form of versioning and protection against accidental deletion, but might have very limited version recovery options. Backup solutions tend to offer straightforward recovery of deleted and older versions of files.

Loss of directory structure. Typically an online backup service will backup your entire User directory (if not your entire PC), with structure intact. That makes it easy to recover onto a new or freshly formatted PC: just copy the User directory over and all the files within it – photos, documents, save games, application settings, videos – will be restored to their proper place and for all users. It’s not so easy with a sync drive. You commonly cannot just sync your User directory since it’s not writable, so you have to individually select subdirectories (eg. Documents) to sync and then reconstruct them later.

Can be worked around by ransomware. A ransomware infection will encrypt your important files, including the files in your sync directories. Those encrypted files might then be synced with your drive, overwriting the real files.