Devil's advocate: using an Internet-public domain for internal purposes will publicly expose your internal hostnames if you enable DNSSEC on the Internet-public domain. This is a problem if you're required to enable DNSSEC, e.g. for FedRAMP compliance.
The number of cases where this is actually a legitimate concern, IMO, is extremely small, and I'm personally of the opinion that using Internet-public domains for internal purposes is generally fine. But it's still important to point out that the number of cases is not zero.
No it wasn't! NSEC3 is crackable the same way a 1990s Unix password file is. This was such a big problem that two competing approaches were introduced to defeat it: "whitelies", which I perceive as the "best practices standard" answer, requires servers to operate as online signers (they should have been all along) so they can generate dynamic chaff records to foil enumeration, and NSEC5.
The number of cases where this is actually a legitimate concern, IMO, is extremely small, and I'm personally of the opinion that using Internet-public domains for internal purposes is generally fine. But it's still important to point out that the number of cases is not zero.