Obviously I defer to KB’s knowledge on this, but I am surprised that Box, which has always seemed one of the more Mac friendly sync services, hasn’t come up with a decent set of APIs for syncing with iOS. Perhaps they have but they simply don’t lend themselves to the scriv package format.
AFAIK they don’t have a device limit and do offer 10Gb basic, although the maximum file size is limited.
Up-thread I had said I hadn’t tried in a few months this method of transferring between Mac and iOS:
https://forum.literatureandlatte.com/t/ios-11-support-of-files-app/37322/19 (KB Oct 02, 2017)
After that post I tested* using the method and have successfully transferred a Scrivener live project from iCloud Drive to iOS. If there’s one existing copy of the project in iOS Scrivener, the transferred in copy will be appended with a dash + 1. While you can’t transfer a live project from iOS to iCloud Drive, you can easily transfer a time-stamped ZIP file to it. Just another way, among a few others, to get the job done in Scrivener working cross-platform.
*Tested using macOS and iOS. Didn’t test Windows.