> Is there a way to make my arrangement more robust so that I can sync files through and USB drive, without needing to use the same numeric user id in all my computers?
Yes, you very likely need to be root. You could also perhaps add in fstab that you mount the USB drive as a specific uid. See man mount and man fstab and search for " uid" (with the space, else you search for uuid as well).
$ chown -Rf ufo.users /media/usbdrive && unison