FTR, Paul is talking about a hostname like "generic_hostname_automatix.foo.bar.lan"

As replied in a personal mail:

the "_" shouldn't be used in hostnames as it might cause issues, see
RFC 952 -> https://tools.ietf.org/html/rfc952 - and "." serves to
delimit components of "domain style names". So grml-hostname is
rejected it nowadays to avoid issues during either invoking
`hostname` (which will complain then with your hostname, at least
nowadays and on Debian) or cause issues in production.