May return false under the following conditions:
The file you're trying to write is larger than 100MiB as defined by k_unMaxCloudFileChunkSize.
cubData is less than 0.
pvData is NULL.
You tried to write to an invalid path or filename.Because Steamworks Cloud is cross platform the files need to have valid names on all supported OSes and file systems. See Microsoft's documentation on Naming Files, Paths, and Namespaces.
The current user's Steamworks Cloud storage quota has been exceeded. They may have run out of space, or have too many files.
Steamworks could not write to the disk, the location might be read-only.