> The parts of it being uploaded to iCloud, not the whole thing, yes?

This is policy, not capability. The capability is each and every image can be scanned.

> As I understand it, the whole point is that the hashes are computed on-device and compared to the NCMEC database on-device. Is my understanding incorrect?

You are incorrect but also the concern is NCMEC’s database’s accuracy. Also, going by other NPO’s the government has taken an interest in, NCMEC is very exposed to government control. They can easily decide to allow the government to slip a few extra hashes in their database and tell no one. They can do this in exchange for protecting their NPO status, a national security letter, anything. We will never know and if we find out, we will have no way to hold them accountable. Ever.

Anyway, the NMEC database is NDA’ed and a secret. It’s not open to review or auditing. It’s also grossly inaccurate and nearly useless.

You are taking these points entirely out of context and in a vacuum. It reads a bit bad faith and win an Internet argument at any cost.