iCloud is a useful storage extension on macOS as it shows as a location on Finder. However, imo I don't think the full benefit of iCloud can be enjoyed outside Apple devices. It has limited usefulness on Windows, and zero on Android. So I would only opt for iCloud if one really lives inside Apple ecosystem. If one uses multiple platforms, services like Google Drive or OneDrive might be more practical.
I think where these iCloud storage threads get off track is the definition of additional or extension to storage.
iCloud is not similar to other online storage services due to it not having granular means to control which files are stored in the cloud and which ones stay on the local drive under all circumstances and by control of the location by choice of the user.
By enabling iCloud, it is simply moving local folders to /Users/<username>/Library/Mobile Documents. In that folder is the root of iCloud Drive, and also Documents and Desktop if that is chosen to by synced by the user. All files get copied/synced to Apples iCloud servers, and also remain on the local drive unless space runs low.
If the option is chosen, iCloud will chose to delete files off of the local drive when space runs low, allowing newer files to created thereby storing more than that of the local disk size. All this is transparent to the user and at the same time takes away control from the user on which files to always have present on the local system.
Many people want additional storage to what they have, and don't want their files removed off of the local system, as iCloud will do. iCloud is a sync service. Anyone that just wants to remotely store some folders, or store folders on a drive other than on the local system need to understand how iCloud works, because to a lot of people having a service manage where their files are located is terrifying, in that they lose control.
