There seem to be reasons that a court might find the Stormy Daniels NDA to be non-enforceable. I am listing some and would like to know if there are examples that support a court rejecting such NDA agreements. Payment was made to conceal something that the public has a right to know for national security reasons. Payment was made in a criminal manner or to conceal a crime. A person agrees to something that they have a constitutional right to do, such as get married, or in this case speak about their rulers. I have this sense that our founders wanted the first amendment to protect us from abusive rulers and dictators, and I can even envision a place where Donald Trump says he paid everyone in the USA ( via a $1 rebate on their 1040) to never speak against him.
In the end, an NDA is just a contract, and you are asking whether the contract is enforcable.
The contract said that X pays an amount of money, and Y doesn't tell facts that are true. Y signed the contract, X apparently didn't sign but paid the money. Both X and Y clearly expressed their intent to be bound by the contract, X by paying the agreed money, Y by signing the contract.
In my opinion, that contract is perfectly legal and enforceable. And if anyone could claim it is not valid, that would be X whose signature is missing, but not Y. Not that I think X could get their money back.
The contract is not enforceable even when considered with the side letter, which identifies the parties and, importantly, specifies whose confidential information is protected. The actual agreement is between Trumps lawyer (represented as a separate company)and Stormy. Trump is not a party. This is problematic as the nda contains legal promises made by DD. So he would need to be a party as Trump’s lawyer cannot guarantee performance and Trump cannot be held responsible for promises he did not sign off on.