The proponents of Prop 65 had good intentions. It sounds good on paper. Drag everything out into the light, and let people make educated decisions. However, it has become yet another case of information overload. When almost everything you buy (slight exaggeration) has a Prop 65 warning label on it, the signal-to-noise ratio is so low that the warning labels are effectively useless.
Not to mention the stupidity of representing carcinogenicity as a boolean. So even the most carcinogenic substance on the list gets the exact same Prop 65 warning label as the least carcinogenic substance. In terms of your actual risk of developing cancer from said substances, that's a pretty huge range. At the high end, you have the usual things like industrial solvents, etc. And at the low end, things like food coloring that has only been shown to cause cancer in lab rats getting massive daily injections of the stuff.
And of course, the whole thing is premised on the idea that people are going to see the warning label, decide to go do some research, and then make an informed decision. That's laughable. The vast majority of Californians (including me) just ignore the labels entirely. Most of the rest probably just get scared off of buying anything with a Prop 65 label on it. Maybe there's some really tiny percentage of people that actually do the research, but those are just the same people who'd be doing it anyway, and I sincerely doubt that those people find the warning labels to be useful, since you basically just have to research everything.