I can only speak from the POV of my country (left unnamed), but I encourage anyone wanting truly interesting work to apply to your country's SIGINT agency. Only there will you find real answers.
.. which it's illegal to tell anyone else. And if you find evidence of abuse the only way you can do anything about it is to flee to a country from which you can't be extradited.
This is why we can't have a discussion about this and as much as possible needs to be moved from "intelligence" (secret, unaccountable) to "evidence" in courts.
The only sensible answer to "how many terrorist plots have been prevented" should be a count of convictions.
Not true.
https://en.m.wikipedia.org/wiki/William_Binney_(U.S._intelli...
At the July sentencing hearing the presiding judge, Richard D. Bennett of the Federal District Court, issued harsh words for the government, saying that it was "unconscionable" to charge a defendant with a list of serious crimes that could have resulted in 35 years in prison only to drop all of the major charges on the eve of trial.[61] The judge also rejected the government's request for a large fine noting that Drake had been financially devastated, losing his $154,600 job at the NSA and his pension.
Sounds so easy.