Hi,
We had encountered an issue saving a CKShare which uses a larger parent hierarchy of more than 5.000 records (see https://forums.developer.apple.com/thread/64194)
Sadly, we just received an answer from the (very helpful and responsive) Apple Developer Technical Support that CKShare is currently only designed to share a few hundred records. The hard limit seems to be 5.000 records when creating a CKShare, the recommendation is 200.
Be sure to design your apps with these limits in mind when you're planning to use CKShare. It would be great if anyone with a real-world usecase for an app can file a feature request (http://bugreport.apple.com). Mine is #28548349.