The thing is, Goldberg seems to advance a set of theses in his book without clearly distinguishing them. From his blog it's apparent that he thinks the claim that fascism is left-wing is the primary or central claim of the book. (It's also the one even sympathetic reviewers, like Michael Ledeen, find most unconvincing.)

But in the last few chapters he leaves off intellectual history and begins a sort of guilt-by-association spree: the Nazis were obsessed with organic food! That seems like a dangerous game for him to play as well. After all, which side of the American political spectrum frets more about declining birth rates?

Then there's the stuff about how the progressive movement in the US did a lot of nasty stuff while praising a lot of the nasty stuff Mussolini was doing. It would have been nice if he had just focused on this thesis, since it happens to be actually true.

And finally, at the very end, there's the admission that the sort of "guilt-by-association" stuff can be done against conservatives too, with respect to "national greatness" conservatism and the rest. We are all fascists now and all that sort of thing. Well, fine, but that and similar qualifications spread throughout the book seem to muddy some of the previous chapters so much that it's hard to even extract an argument from them any longer.

So I think the difficulty of arguing against Goldberg stems more than anything else from his sloppy argumentation. The "fascism is left-wing" trope can be done a lot better. In fact, it has been, by people like Kuehnelt-Leddihn.