The best way to ensure you have a full backup with this method is to not enable any optimization until you have your first complete backup performed on all the services you plan to use. That will leave you with an archived copy before macOS could decide to remove the file for optimization.

You can “game” optimization in one specific way, though you can’t guarantee it will work to back up all the files you want: the most recent files you create or modify will almost always be backed up quickly by Time Machine or an online service that frequently checks for modified files. It’s unclear what happened, but these synced files so far remain missing. Even with Apple’s help, they were able to restore only a fraction of the lost files, despite having experienced going through this before: in a previous failure, all files were restored.

Inexplicable failures: One Macworld reader recently reported that their daughter lost a number of music files, despite having Sync Library enabled in Music and an iTunes Match subscription.

With a local Time Machine volume, it might be hours.

Time to restore: Unless you have a high-speed, uncapped, no-overage-charges internet connection, if you have to restore your files by resyncing to iCloud with a new Mac or a new drive, it could take days or weeks with hundreds of gigabytes of data.

Both iCloud Drive and iCloud Photos have Recently Deleted collections (a folder and an album, respectively), but their contents empty automatically after 30 days.

Delete items and don’t realize it until it’s too late: iCloud doesn’t retain deleted items for long.

(You can mitigate some of this risk by enabling iCloud Recovery Data Service.)

Lose access to iCloud: You could be locked out, have equipment stolen, have your account hijacked, or be blocked by Apple, and, despite the lack of legitimacy involved, never recover access.

Making iCloud your only copy for synced data that’s pushed off your drives or devices for optimization has drawbacks: The drawbacks of relying on iCloud-only offloaded files

How to backup mac to icloud mojave