Secrets and Lies
One problem with secrets is that they give rise to lies, like the director of National Intelligence lying to congress in open hearing: here Of course it might have been less humiliating if he didn't have such prominent "tells."
The late Edward Teller, a prominent critic of excessive secrecy, pointed out more serious problems. Secrets may hinder you more than your rivals. That certainly applies to technology secrets. Teller liked to cite the example of the (then) secret trick behind the H-Bomb. When we discovered it, noted the "father of the H-bomb," we knew that it was so cunning and unexpected that nobody else would think of it in a hundred years. A few months later, he noted, the Russians exploded their own thermonuclear device. The lesson was, he said, that what Americans could think of, Russians could too.
Do either of these lessons apply to stuff like the UK and other countries collecting data on who their citizens and others talk to? Maybe not, but as we have seen, the more or less inevitable exposure of such programs has its own bad consequences. One hardly need mention the potential for abuse in the hands of the unscrupulous.