For context, I deeply hate the abuse of children and I've worked on a contract before that landed 12 human traffickers in custody that were smuggling sex slaves across boarders. I didn't need to know details about the victims in question, but it's understood that they're often teenagers or children.
So my initial reaction when reading this Twitter thread was "let's get these bastards" but on serious reflection I think that impulse is wrong. Unshared data shouldn't be subject to search. Once it's shared, I can make several cases for an automated scan, but a cloud backup of personal media should be kept private. Our control of our own privacy matters. Not for the slippery slope argument or for the false positive argument, but for its own sake. We shouldn't be assuming the worst of people without cause or warrant.
That said, even though I feel this way a not-small-enough part of me will be pleased if it is deployed because I want these people arrested. It's the same way I feel when terrorists get captured even if intelligence services bent or broke the rules. I can be happy at the outcome without being happy at the methods, and I can feel queasy about my own internal, conflicted feelings throughout it all.