Schneier explained how, initially, NSA Director General Keith Alexander claimed in 2013 that he had disrupted 54 terrorists plots. A few months later, this was revised down to 13, and then to "one or two." Eventually, the only success that the NSA could point to was the prevention of a San Diego man sending $8,500 to support a Somali militant group.
Doesn't sound like a worthy trade-off.