While maybe literally true, "risk that you cannot understand" is a little overthreatening. Are you implying extrajudicial financial ruin, torture, murder? I doubt it, so what?
The point being made upthread is that there is no black and white guidance, there is no secret NSL Guide Book for attornies.
The FBI does in fact has some guidance that your attorney will be able to locate[1]
[1]: https://vault.fbi.gov/National%20Security%20Letters%20%28NSL...