When the scan is carried out on device, Apple can't read the results of the scan until the 30 image threshold is reached.

When Google scans on server, a single false positive result can be abused by anyone who can get a warrant.

>Innocent man, 23, sues Arizona police for $1.5million after being arrested for murder and jailed for six days when Google's GPS tracker wrongly placed him at the scene of the 2018 crime

https://www.dailymail.co.uk/news/article-7897319/Police-arre...

Apple's method is more private.