The original proposal was to use a visual hash designed to identify known bad CSAM images even if cropped or otherwise edited. Your computer would scan all your stuff for these images and report you to apple who would report you to the cops. This presented a number of issues.

Accidental false positives could lead to horrific outcomes up to and including oh look bob got shot by the cops for resisting.

It was possible to produce apparently matching innocuous images and then poison people's machines with them.Oops did you click on that picture of a tree have fun with the cops. Like an advanced form of swatting.

Although inspired by a desire to find CSAM Apple could be forced to scan for ANYTHING by repressive regimes including America and China.

Although initially targeting images client side scanning of messages is a pretty obvious next step. Again obvious good motivation exists and is completely justifiable who doesn't want to stop the next mass shooting or terrorist attack... and then we can basically use it to find people critical of the regime. Do remember we are presently prosecuting a political figure for a picture of sea shells and a guy in texas is rotting in prison for distributing political literature.