When uploading 10k photos from macOS to Google Cloud using the Google Cloud macOS app, it said syncing had completed about 2 hours earlier than my back-of-the-envelope calculations predicted. "Great", I thought, but was still a bit suspicious, so just in case, before deleting the local copy, I closed the Google Drive app and reopened it, and it immediately started syncing - there were 2k photos/videos to go (!!). That's how insanely easily it could be to lose precious memories due to a tiniest bug in cloud software.
Surgical precision and extreme thoroughness are the only ways to approach these seemingly simple operations of moving files from one computer to another.