What are the benefits? It would tell Apple (and hence law enforcement) who had copies of existing, known CSAM. I guess that would help catch such people. But such individuals would also have strong incentive to simply turn of iCloud to completely avoid detection.
What are the risks? Apple employees rummaging through your photos (after 30 NeuraHash matches, a human reviewer would go in and check your photos, and as some analysts pointed out, hash matches can be spurious, that is, completely different photos can have the same hash by coincidence).
Over time, would the program expand beyond CSAM? Consider the intersection of countries in which Apple sells iPhones and countries that criminalise being LGBTQ [1]
[1] https://www.humandignitytrust.org/lgbt-the-law/map-of-crimin...