NDAs (and non-competes) are pretty much never worth the paper they are printed on. They are genuinely unenforcible contracts in almost all cases.

The only real power they have is that, should you violate them, the entity you signed them for can sue you. They'll almost always lose if it goes to trial (globally they almost always do) - but that will be a very large amount of money and time later - and a lot of legal fees.
Most people can't afford a legal fight with a government department or large corporation so the vast majority are settled out of court.

And because people know that, most people obey NDAs and non-competes because it would be too expensive to deal with the difficulties (and no guarantee) of showing the contracts are not, in fact, legal.

But as a standard rule - never sign an NDA because you can only get screwed by a contract that's got all the legality of used toilet paper.