The fisa abuse is something that should never have happened. That's an area where the law says "trust us" since there's no oversight. Plus the judges are on the federal bench and considered above reproach.
The fact that these secret courts don't include an adversarial component is the big problem. Our normal judicial system is based on two sides making arguments. In the FISC system there is just one side presenting "evidence" to be evaluated. Throw in a corrupt investigator or two and viola, big problem. And that's not to say that everyone involved in the chain of evidence or who signed off on the applications is compromised, but if the "evidence" is already in place, how do they know if it's verified and accurate? They're simply following the evidence as presented by the investigators.
It's a real problem, regardless of those who can't or won't see it.......... and I have no idea how it gets fixed. But the first step is acknowledging the problem exists.