I'm on High Sierra 10.13.6 on a MBP 13" 2015 and iOS 11.4.1 on an iPhone 7. I have been using iCloud for several years and have never had any trouble with it.
However a few days ago, I noticed the Bookmarks and Favorites weren't syncing from my iPhone to my MBP, and when clicking on "edit" on iPhone's Safari Bookmarks I constantly got the message:
Bookmarks are being synced. Please add the bookmarks once syncing has completed
I have tried setting off Safari iCloud sync on both devices, also deleted permanently bookmarks from both of them, deleted the file "Bookmarks.plist" from ~/Library/Safari and also deleted the file "com.apple.SafariCloudHistoryPushAgent.plist" from ~/Library/Preferences.
All that with no luck. The deleted bookmarks keep showing up on iOS Safari when turning back on Safari iCloud Sync on iPhone.