One of the arguments is that the relative price of gold is too volatile. That strikes me as the wrong argument to make. For one thing, if you had a gold standard, then the relative price of gold might behave differently than it does now. For another, I think that the main Keynesian argument against a gold standard is that its relative price is not volatile enough. In particular, the argument would be that wages tend to be fixed in monetary terms, which under a gold standard means that they tend to be too sticky in terms of ounces of gold.
I would have answered the question of whether price stability and employment would be better under a gold standard as "uncertain." I am surprised that none of the economists took that view. I wonder if they are comparing what they think would happen under a gold standard to what they think should happen with (well-designed and well-executed) discretionary monetary policy, as opposed to what does happen. A case of mental substitution, perhaps?