Can I use iCloud Drive as a backup with non-macOS?
No single backup destination should be your only backup of important files. iCloud Drive is no exception.

iCloud Drive is supported on Windows as you've found. This folder is synchronised with iCloud Drive, so providing you have an internet connection, the files you add there will be copied to iCloud. Later, on another computer or if you erase your current computer, you can sign in to iCloud again and your files will still be there.

iCloud Drive is not meant to be a backup solution for your files in their current location. You shouldn't expect to store all your files in iCloud Drive on Windows as the integration just isn't there yet. You can add iCloud Drive as a library folder so it appears as part of Documents, but unlike macOS you can't replace your entire Documents folder on Windows with iCloud Drive.

An external drive is useful for backups since you take the files on your computer, copy them to the external drive, then remove the drive and place it elsewhere — that's a quintessential backup. iCloud Drive is a little different:


If you copy all your documents into iCloud Drive, you're using twice as much storage space on your computer.
If you move all your documents into iCloud Drive, they're no longer where you left them in your Documents folder or wherever.


iCloud Drive is great for files you want to synchronise, and can work as a backup solution, but it's not designed for that primarily.