This article[1] is a good start at realizing the costs outweigh the benefits. There's little or no evidence of good caused, but plenty of evidence of harms caused.
[1]: https://www.eff.org/deeplinks/2014/06/top-5-claims-defenders...
So there's your control group, Europe.
This is on top of all the entrapment that we also know about, performed by the FBI and associated informants on Islamic/Muslim communities
The purpose of a system is what it does
In comic books, we read fanciful stories about the good guys saving the world in secret. But the real world doesn't really work like that.
When the police seize some illegal drugs, what is the first thing they do? They snap a picture and publish it for society to see:
https://www.google.com/search?q=police+seize+drugs&tbm=isch
because citizens want to see that their tax money is being used successfully. The same would likely be done by the surveillance authorities if they saw significant success in their mission.