Dropbox is still the recommended sync platform and is required for syncing to iOS devices. If you’re just syncing between two computers, you can use some other services like OneDrive and Box. You can check out any active advisories we have on platforms with known issues here.
Scrivener projects are actually not compatible with iCloud for syncing purposes. If you use iCloud to go between devices, you will eventually lose data. It’s fine to keep projects in a folder that backs up to iCloud but this should not be used to go between devices. You can read about future iCloud compatibility here.