iCloud was introduced many years ago, however, developers still see issues with syncing data in their apps using Core Data.
NetNewsWire, Overcast developers (as well as others) share their thoughts in couple of blog posts.
Most developers report long syncing time and overall unreliability of the service. Moreover, according to internal app statistics, it sees that about 10% of users do not have iCloud meaning that they did not entered their Apple ID in system settings.
Overall recommendation to developers who plan to use iCloud is to have an alternative by either having their own sync solutions (self-hosted or using some public cloud offering), or rely on third-party as data storage (NetNewsWire allows to use multiple RSS aggregators in addition to iCloud).
Developer:Go through developer feedback, consider using additional safeguards and analytics to understand if iCloud sync works properly for users.
QA engineer:Always test iCloud sync between different app versions, verify sync between different platforms, if applicable.
PM/DM:Beware of possible issues, be ready to have alternative syncing mechanisms developed.