I don't think it's going to be the big deal people are hoping it's going to be. She hasn't been very forthcoming so far and is still bound by the agreement to keep quiet, at least until the agreement has been deemed null by the courts. I'll be very happy to be mistaken though.
There's no requirement to keep quiet about a threat.
However, what would surprise me is if there was no threat. Some trumpet probably would have made threats. Doesn't mean it's a serious threat.
More so, no NDA can legally cover any illegal act. If the payment to Stormy was illegal (due to being deemed a campaign contribution - since its direct and sole purpose was to benefit the campaign), then that aspect of the agreement is void. She can talk freely about receiving illegally allocated funds. Further, in the state of NY, where both the "incident" and the paperwork was signed (by only one party), Adultery is technically illegal. Therefore, everything about their encounter with each other cannot be covered by any type of NDA, since the encounter was an illegal act itself.
I am certain that the NDA will be deemed invalid. Also, I am fairly certain that the money paid to her will NOT be required by the court to be repaid, since the vehicle to accomplish that would entail the admission of more illegal activity on the part of Trump... so, possible, but highly unlikely.