One concern I've read is that safety testing is done in isolation, and glyphosate is actually used with a surfactant, designed to make it able to penetrate cell walls.
The whole is different than the sum of its parts.
But is that relevant? Isn't it more sane to let our first assumption be that plant killers aren't good for us? Until we have some compelling reason to believe otherwise?
Judgment aside, there's a cost for the society which is not borne by owners of these giant corporations. These owners have a selfish interest to propagate dubious means. With "selfish" I mean the good natured selfishness in the sense of Smith's capitalism. So we as a society should strive to charge the owners with these costs, so that products like RoundUp get uneconomical to produce and sell.
Keyword: external costs
As an aside, I think the conventional wisdom reading of Adam Smith, that somehow selfishness begets virtue, was not the author's intent, and is mostly just a self-serving rationalization. At the very least, it ignores the ways that selfish action can turn toxic - deception, corruption, theft, and of course externalized costs.