Nice post by Rich Siegel of Bare Bones Software:
Core Data syncing: This is where the rubber meets the road for database-backed applications. Core Data is the application-level database framework supplied by OS X and iOS that provides the means for applications to store items, and data about those items, in a single database. Yojimbo, our product, was one of the very first to ship using Core Data storage — we’ve used it since 2006, and it works great for storing data locally in the way that the product needs it to. Syncing database changes with iCloud, however, is a very complicated and difficult job for Core Data.