I think the perceived problem is the database of hashes to be matched. If this is a database that can be contributed to by other parties this can be used to "frame" people of a crime. This can the be used for example to get rid of political dissidents etc. Since there are just hashes in the database, this should be fairly easy.

This is what I THINK people are worried about. I don't have an Apple device so I haven't really fact checked all of this.