As someone who works on integrity for a small ad platform, I can tell you that there is no way to prevent 100% of the bad things from going through. We are tiny and still deal with highly motivated bad actors.
Fwiw, Google has also had a problem with this and has paid half a billion in fines.
https://www.businessinsider.com/google-loophole-buying-drugs...
That makes certain assumptions about technology and business model? If humans reviewed every ad, the way newspapers once reviewed classified ads, then you could detect near-100% of them.
Somehow tech companies have convinced everyone that they have a right to automation and an associated business model and profits, and that the right trumps even criminal activity.
Obviously, overwhelmingly - based on massive illegal sales (not just drugs, not just ads), propaganda, fraud, scams, horrific abuse of children [0] - that technology and business model have failed completely.
[0] https://www.washingtonpost.com/investigations/interactive/20...