I think this should be paired with a sanity check in snapcraft that the user running the snapcraft command has umask 0022. My umask is 0077 and therefore I just snapcrafted a package successfully, that was then rejected by snap install:
"meta" should be world-readable and executable, and isn't: drwx------
"meta/snap.yaml" should be world-readable, and isn't: -rw-------
It was not immediately obvious to me what I did wrong. Better to prevent such things at snapcraft-ing time.