A post on dpreview makes the claim:
UV haze and high contrasts in sunny days will make any good lens a poor bokeh performer and bokeh will likely be fuzzy and chaotic. Bokehs [sic] look best in controlled, subdued light [...].
Now, I can buy the argument about high-contrast sunlight. This seems likely to bring emphasize harsh bokeh — although, a question remains: would it make nice bokeh worse?
But, I'm unclear about the "haze" point — no pun intended. Wouldn't any atmospheric haze have the opposite effect?