Unlike Parse, of course, Apple’s solution will not be cross-platform. This will make it unviable for a lot of developers right off the bat. That being said, there are lot of indie developers (myself included) who are single-platform shops. Assuming it is good, I think a reasonable portion of developers would adopt it. Given Apple’s history with cloud services, that’s a big ‘if’. For instance, developers wanted to use iCloud with Core Data, but ended up shying away from it because of its numerous flaws.
Also, note that some cross-platform apps could still take advantage of something like this. Games for example, despite being multi-platform, aren’t expected to sync save state across iOS and Android. They could still use this system, for syncing across a user’s iOS devices.