The main problem will be services that assume at least one of the NIST time servers is up. Somewhere, there's going to be something that won't work right when all the NIST NTP servers are down. But what?
If you have information on what they actually are using internally, please share.
(Yes, I've heard the Ravel before, I mean the presentation style, e.g. Oldfield: https://www.youtube.com/watch?v=QdMtqKZ6GrY )