On the topic of backing up data from cloud platforms such as Onedrive, I suspect this is stop the client machine from actively downloading 'files on demand' which are just pointers in explorer until you go to open them.
If you've got huge amounts of files in Onedrive and the backup client starts downloading everyone of them (before it can reupload them again) you're going to run into problems.
But ideally, they'd give you a choice.
This is a pain, to be sure, but surely there is some sort of logic you could implement to detect whether a file is a Real File that actually exists on the device (if so, back it up) or a pointer to the cloud (ignore it by default, probably, but maybe provide a user setting to force it to back up even these)
It used to be the case that placeholder files were very obvious but now OneDrive and iCloud (possibly others) work more like an attached network storage with some local cache, and that was a good move for most programs because back then a file being evicted from storage looked like a file deletion.
Came here to say this. Files in OneDrive get removed from your local storage and are downloaded ON DEMAND. given that you can have 1TB+ onedrive folder, backblaze downloading all of that is gonna throttle your connection and fill up your disk real fast.
No reason for that to be true.