Perhaps the best interface for a cloud data storage provider is a block device, randomly accessible by block. It is up to the user to format this network block device to something useful, like a filesystem with whatever features a user desires.
I know of no cloud data provider which does this. They all try to be more user friendly, which makes things complicated and difficult.
Simultaneous access remains a thorny problem, though it is squarely the user's (or users') problem now. The network block device could provide some atomicity primitives.