There are certainly all sorts of body-harvesting abuses available; but they are essentially orthogonal to whether you operate on an opt-in or opt-out system.
If you are willing to kill someone for their organs you clearly don’t have a lot of interest in honoring their consent, as well as having enough impunity or obfuscation to feel safe in running that sort of operation. Nobody in that situation is going to have much trouble ensuring that the record(if there has to be one) has an adequately plausible ‘consent’ placed in it, one way or another.
The only case where it makes a difference is among the apathetic.If you have to go out of your way to donate you’ll get fewer donors than if you have to go out of your way not to. Anyone who proposes an opt-out scheme that’s clearly designed to beat people down by attrition is an issue; but real-world implementations usually make it as easy as a checkbox on some form you would be filling out anyway, with the additional option of a declaration to that effect if you are still on speaking terms and end up in the hospital.
