There is, indeed, general agreement that the price-earnings ratio of trading banks is lower than that of other banks of the same size, although it has been difficult because of the dynamics of other variables to nail down empirically the appropriate orders of magnitude.
And, I suspect, that if the data were readily available, we might be able to demonstrate the same pattern at institutions significantly involved in the private equity market and perhaps even in derivatives trading. Any earnings stream that shows variability has been appropriately discounted.