The test phase at the end of the compose should validate that all bootable image have the correct bits so that they can actually be booted.
If any image is broken in this way, the compose should be marked as failed.
This would prevent problems such as https://bugzilla.redhat.com/show_bug.cgi?id=1331317
This issue has been migrated to Fedora Forge: https://forge.fedoraproject.org/pungi/pungi/issues/288
Please continue any further discussion there.