Looks like Nextcloud's synchronization program said everything was synced but actually dozens and dozens of my photos from the last few years are zero length files.
When attempting to download my nextcloud files on a different machine, I installed the client app and it says sync is done/finished/ok, but it's only synced down 40GB from the server which claims to have 300GB of data, also.
Don't use Nextcloud, don't trust PHP developers, don't trust Euro data hippies.