My iPhone6+ got stuck when uploading some photos automatically to iCloud Photo Library. It’s quite possible if the phone’s available free space is lower than 400MB when using optimizing size option, which might be required by the mysterious blackbox photo upload process. But after cleaning up disk space, it still paused at the “uploading xx photos” phase.
Tried everything googled: reboot phone / disable-enable iCloud Photo Library / delete videos / logoff iCloud account, nothing worked.
Finally, forget about the iCloud synchronization. Disable Photo Library, transmit these photos to Mac and manually upload photos to iCloud.com, then delete all existing photos from the phone. And at last re-enable the iCloud Photo Library to download all photos again to the iPhone.
Hope it will sync next time.