Prop 65 cancer labeling did actually reduce the incidence of many of the more potent carcinogens in products when it was first enacted. So in that sense it was very successful.

But because of the wording of the measure (no penalty for falsely labeling something as having carcinogens when it doesn't), and how many things can be considered carcinogens (caffeine is just one example), the effectiveness of the label was diluted dramatically in people's minds.