Dropbox and onedrive can handle backblaze zipping through and opening many files. The risk is getting too many gigabytes at once, but that shouldn't happen because backblaze should only open enough for immediate upload. If it does happen it's very easily fixed.
If it overloads nextcloud by hitting too many files too fast, that's a legitimate issue but it's not what OP was worried about.
The issue you’re missing is that the abstraction Dropbox/OneDrive/etc provide is not that of an NFS. When an application triggers the download of a file, it hydrates the file to the local file system and keeps it there. So if Backblaze triggers the download of a TB of files, it will consume a TB of local file system space (which may not even exist).
It does keep them permanently. Dropbox is not a NAS and does not pretend to be one.
> When you open an online-only file from the Dropbox folder on your computer, it will automatically download and become available offline. This means you’ll need to have enough hard drive space for the file to download before you can open it. You can change it back to online-only by following the instructions below.
Same exact behavior for OneDrive, though it apparently does have a Windows integration to eventually migrate unused files back to online-only if enabled.
> When you open an online-only file, it downloads to your device and becomes a locally available file. You can open a locally available file anytime, even without Internet access. If you need more space, you can change the file back to online only. Just right-click the file and select "Free up space."
Dropbox and onedrive can handle backblaze zipping through and opening many files. The risk is getting too many gigabytes at once, but that shouldn't happen because backblaze should only open enough for immediate upload. If it does happen it's very easily fixed.
If it overloads nextcloud by hitting too many files too fast, that's a legitimate issue but it's not what OP was worried about.