Now that the technology is on-board the device, how many lines of codes do you think it will take to scan the full photo-roll?
Do you think that this ability will not tempt LEA, law makers, governments, to push for that ever-so-small change to the code, either for blanket monitoring (see if China is not tempted, using their own database of "illegal" content) or for targetted monitoring (some specific users, with or without valid court orders).
The main issue is that the wall has been breached: monitoring data that was otherwise only on-device is now possible with little to no change as the feature is now embedded in the OS.
You can argue we're not there yet and can trust Apple to do-the-right-thing and that the Rule of Law will protect citizen against abuse, but that's a big step into a worrying trend, and not all countries follow the Rule of Law and have checks and balances to avoid misuse. Don't forget that Apple abides by the laws of countries where it sells its devices. That means they will -forced or not- do what they are told.