Even after reading everything I could find on this, reading through the few threads here on this, I am still left with a couple of nagging questions:
1. Why has Apple chosen to scan this way?
2. Why now? What is the over-riding reason for this now?
For #1 I have not found anything that remotely explains this. Why not do it the same way everyone else is; server side.
For #2 I wonder if the push to scan is a result of regulation the EU is driving.
For #1, it's so they can implement this CSAM "protection" while also claiming they still can't access your photos. If they scan them on iCloud, they have to decrypt them, which opens the door (pun intended...) to being able to access/view all of your photos. Other companies simply tell you, "Hey, you're uploading to our servers, so we can do this." Apple, to their credit (or grandstanding?), doesn't like that solution. The way they're implementing this allows them to basically say, "Hey this is going to iCloud anyway, and this keeps us from seeing all of your photos, and only puts the questionable ones in front of us."
The irony of this is in order for them to be able to claim they can do this while not accessing your photos, they've essentially created a way for others (and themselves if they needed) to access and eventually, if need be, see things that were formerly protected by E2E. In the process of saying, "Hey we can do this without seeing your encrypted stuff", they've decreased one of the primary benefits of E2E.
#2 - From everything I read, Apple truly was doing the least out of all companies in this regard. So either they felt like they created a solution that was acceptable from an encryption standpoint, or they were getting pressured to come up with this "solution" (see #1 again...). Or maybe a little of both.
Honestly I don't question the accomplishment they've made here, and there is precedent for me to trust that they're doing it with the proper motivation. I simply think it's penny wise and pound foolish, and don't like them having to use my device to do it.