Were they used for two decades without strong evidence of problems while people were looking for such evidence?

They look to me like things that no one had any idea were problematic -- until at some point evidence of trouble started to appear, and fairly quickly it became consensus that they were bad. (Possible counterexample -- I'm not sure of the dates -- is cigarettes, only because there was a very well-funded systematic disinformation campaign conducted by the tobacco companies.)

In the case of GMO, some people have worried (publicly, vocally) about safety from the very beginning. That's quite different from lead pipes or asbestos or even cigarettes.