If they're canvassing for witnesses, are they going to charge through your yard and shoot your dog? If they're investigating someone else, how likely are they to try to come up with something unreasonable to charge you with for leverage and then make you plead it down to a penalty that still isn't zero in exchange for giving them information you might not even have and would then be forced to choose between fabricating to get the deal and "not cooperating" and getting a serious prison sentence?
If someone is attempting to SWAT you, how likely are they to ascertain the situation instead of shooting first and asking questions later?
If their investigation has led them to you for some reason even though you're innocent, do you expect them to care about the truth or just railroad you?
If you hear the name of a particular law enforcement agency unexpectedly when you don't have any reason to think you've done anything wrong and your instinct still has to be "oh fuck" then they're bad at their jobs.
Depending on how you expect the reader to answer all your questions, we could still be in full agreement, but my sense is that you're asking them rhetorically?
And then in terms of literal sentiment, most people aren't familiar with any given local law enforcement agency because there are so many of them, so they wouldn't know what to think, and some of them are quite bad. But the knowledge of the average person it isn't really the point.
Suppose you actually were familiar with the record of whatever specific agency just showed up. If you would still have to think "oh fuck" then they suck.
If you ask me, I'd trade the good for enduring the bad.
My shortcut is admittedly a sloppy heuristic (because what else do you have for unknowns like this); for the unmeasurable effects, my bet is that they skew roughly the same as the measurables. For every serial killer who thought twice, there have probably been many political activists who have also thought twice. The deterrent effect cuts both ways if your actions cut both ways. We also know about enough falsely accused / imprisoned that we can assume we ain't figured them all out. For every family that feels safer with the FBI around, there are families that feel less safe, because people "like them" have been framed, murdered, snooped on, suppressed, and criminalized.
So yeah, it is hard to measure - but not impossible to come to a conclusion, as far as I'm concerned.
Another way to look at it is this; if you're going to hand the mandate of violence and skullduggery to an institution, you should be damn sure that they have standards and practices that solidly enforce competence and ethics - and even considering the good, we know pretty conclusively that they have failed in this regard. I don't want to play russian roulette with law enforcement - they should get it right almost all of the time or step aside so someone who knows what they're doing can handle it.
Hard to measure, isn't it. In the eyes of the millions of americans who have at some point in their life been victims or related to or friends of victims of some kind of serious crime, the FBI has often times been helpful and/or the prospect of being caught has been a deterrent for crimes.
You contrast that with all the bad that has come from there, of which there is surely plenty, but how come you claim thay the bad obviously must outweigh the good?