Box Advisory (Older versions)
As of Box client version 4.0.5024 (released Feb. 2021), they now support Mac package formats. The following advisory is thus only applicable to older versions of their software. If you can upgrade to the latest version and wish to sync your Scrivener projects with this service, you should do so.
Box has announced that their service is incapable of handling macOS "package" formats, and that they have no plans to support them in the future. This impacts many different types of Mac files such as RTFD, Keynote presentations and Scrivener projects. Their official position is recorded in this ticket.
While technically speaking a "package" is merely a folder with a file extension as part of the folder's name, and should offer no more resistance to syncing than any other folder, we must defer to Box's advisory and suggest that our users find other services, more capable of syncing such folder-based formats, without malfunction.