The best covert exfiltration is when you can hit individual devices in a crowd, so people already have no reason to be suspicious. But you're still leaving tracks - connections, packet sizes etc. if you actually want to do anything and you only need to get caught once for the game to be up.
This on the other hand is essentially the perfect channel for its type of surveillance...because it is a covert surveillance channel! Everyone is being told to expect it to exist, that it's normal, and that it will receive frequent updates. No longer is their a danger a security researcher will discover it, its "meant" to be there.
Letting Apple scan for government-related material on your device is a slippery slope to government surveillance and a hard line should be drawn here. Today, Apple may only be scanning your device for CSAM material for iCloud. Tomorrow, Apple may implement similar scanning elsewhere, gradually expand the scope of scanning to other types of content and across the entire device, implement similar government-enforcing processes on the device, and so on. It's not a good direction for Apple to be taking, regardless of how it works in this particular case. A user's device is their own personal device and anything that inches toward government surveillance on that device, should be stopped.
Another point made was that government surveillance never happens overnight. It is always very gradual. People don't mean to let government surveillance happen and yet it does because little things like this evolve. It's better to stop potential government surveillance in its tracks right now.
[0] I understand the emissions cheating was not supossed to be open, but a relatively open system allowed said student to take a peek and see what was going on.
Read evidence from past trials it is obvious. See also successful and failed attempts to subpoena this info from VPN services.
Only people with iCloud will be using the relay.
It is true on the surface the photos is disconnected from the use. However, Apple only needs a solid answer that handles the bad optics of what you can do with the Tor-like anonymity of iCloud Privacy Relay.
However, if you look more closely, the CSAM service and its implementation are crafted exactly around the introduction of the relay.
It's better to be forthright, or you risk your valuation on the whims of a single employee.