On a new iPhone7, I’ve gotten the database to mostly sync, although it doesn’t quite finish. The remaining items, for the most part, are larger (between 35-150MB) and many of them audio files though there are smaller pdfs still yet-to-be-downloaded as well.
The app says it’s synchronizing and will sync hundreds of items before finally timing out or whatever happens to interrupt it. But it doesn’t actually download these last items. I can press download on a specific one, I’ll watch the cloud fill up to just a silver left, but it never actually finishes.
I am syncing over Bonjour and have over 200GB free on the phone. I have tried many times. Those tries and the initial sync were, all and all, probably 20 hours cumulative of sync.