I believe the point being made is that those points are effective at reaching people who need little to no convincing and who are already politically supportive. The same points about historical wrongs done to other people unlike them are more likely to alienate the readers who are not currently politically supportive.
To put it another way, preaching to the choir doesn't win you more votes.
I understand the preaching to the choir thing. But a lot of people aren't even aware of the history of the FBI. This isn't even a detailed history, just mentioning some scary and relevant facts about the organization that not everyone is necessarily aware of.
I think that speaking in technical, generic and hypothetical terms isn't effective. You have to ground the conversation in actual misdeeds. Otherwise you're likely to get the "but I don't have anything to hide" response.
The trick tends to be grounding the conversation in the misdeeds of people to whom the person you're talking feels no affinity. If you're talking to an American, use the current Chinese government or the UK as an example of how face tracking can be abused.
Putting an organization that a person has faith in on trial in order to make a political point runs the risks both of getting distracted and of failing to convince in a way that backfires.
You're right! That's a very real and very reasonable concern.
That said, it's fairly uncommon that the choir actually walks away electorally. It tends to require something very dramatic, like Roy Moore. It's far more common that they grumble a bit and vote for whatever anyway, because you committed the sin of advocating for the policies they want while not centering the FBI's racist history.
Again, your concerns are quite reasonable. It's just perhaps possible that what it takes to lose those votes is more than might be guessed.
To put it another way, preaching to the choir doesn't win you more votes.