One of the problems is that it exposed DNS zone information, and one of the reason it did that was it was designed to require signing only once and after that the private key didn't have to be used again until the zone info changes.
(I'm not sure if this is what was being asked, but I'm curious about the answer to that, either way.)
(Phreebird is an online signer, but I don't see why an offline signer couldn't generate these proofs.)
If there is any company in the world actually using Phreebird in production, I'd --- for more than one reason --- like to know about it.
(And indeed, it cannot be done offline - although doing much narrower NSEC/NSEC3 ranges than 'normal' could be done offline).