A lot of what he says in it is uncontroversial stuff that he didn't come up with himself. But you don't get much academic respect for things others have said before you, even if you succeed at bringing it to a new audience (especially if you don't pepper it with source references, which I don't recall GGS doing!)

Being a wildly successful popularizer is always risky for an academic. The most serious criticisms I've seen have been about tangential stuff.