But most of the complaints revolve about the improbablity of a thoroughly evil character (which is not as improbable as it might see, but not what I'm talking about; The Terminal Orc discusses it nicely) or the improbability of a race that is in total agreement.
What strikes me most about it -- and the mirror universe, the parallel world where all the goodies are baddies and vice versa -- is the immensity impracticality of it all. "If I am a lady, and you are a lady, who will slop the hogs?", and by the same token, if I am a thief and you are a thief, what are we going to steal? The mirror universes perhaps have it particularly hard, since the proportions of the honest vs. the thieves would have to be reverse. Always evil races can, perhaps, resort to warfare and slavery. Except that it requires a fair amount of organization to pull off. Even a band of pirates had to have internal peace and harmony (relatively speaking) to work. They knew it, they had their articles. But an Evil race would regard articles as a useful tool to bind others until you were ready to spring your trap, except that they, of course, would also regard them as the same. And everyone would be endlessly looking for ways to bring others down. Even an immensely strong and clever one would have a hard time to keep everyone down at once.
Even without the question of ambition, pride would have a continual effect of abrasion. The orcs that Tolkien portrayed as fighting each other would be only too likely among the wrathful as well. Greed would soon have characters fighting over what there was to be had.
Which is why uniformity would make them even more improbable.