I confirmed that the issue is too much data to be synchronized. If I trim my data set so there are only ~1k entities, the Watch app is able to sync all the data, and the time between CloudKit import start and end events is about ~10s. When the number of entities on the order of ~10k, the Watch app CloudKit import never completes, even after leaving the app running (with debugger attached) for 15 minutes. Is this limitation documented anywhere?

How are you using CloudKit API directly from the complication? My impression was Core Data for CloudKit handles all the syncing and does not expose interfaces to mange it directly. I'm facing similar issues with watchOS app staying in sync with data in CloudKit.

I was getting the error after updating my project to consolidate watchOS app extension and app targets. Adding a WKApplication key to the watch app's Info.plist (type: boolean, value: 1) fixed this for me.