Sure, there is a vulnerability to interception. From a collection standpoint, especially in times of panic and emergency, there is no way to sift through everything, especially if simple countermeasures are used. Trying to monitor the entire body of U.S. communications during a national emergency and looking for certain types of traffic would be like looking for needles inside Rocky mountain sized haystacks.
Also, you'd need to be able to trust the collectors, analysts and administrators. What if they, who have access to a lot more info than most people, decide that something fishy is going on, and they start sandbagging the effort internally. Not to get into detail, but intel guys can be incredibly devious, especially with the home court advantage. It's far too manpower intensive to double check people's work. (I don't mean QC it, I mean literally redo it and make sure it's accurate). Also, you can't just pull people off the streets as replacements.
Think about this, as well. For as long as we've had classified information, people have been sneaking it out and giving it away. It's not like intel guys get strip searched leaving the building. Many in the past have simply hand carried the good stuff right out the front door. In the event of a national emergency of questionable legitimacy, if only 5% were active patriotic sympathizers, with another similar percentage of passive, turn-a-blind-eye supporters, it would be a fatal blow to national secrecy. The system would tear itself apart.