Note
As of MySQL 5.7.18, my-default.cnf is no longer included in or installed by distribution packages.
Source: https://dev.mysql.com/doc/refman/5.7/en/server-configuration-defaults.html
So cp -v $(brew --prefix mysql)/support-files/my-default.cnf $(brew --prefix)/etc/my.cnf in mysql.sh won't work anymore.
I don't know if it's this or something else. Even with an old restored my.cnf the setup doesn't seem to work anymore on a fresh Sierra MBA.