The Klingons should do all right if they keep their heads screwed on. Every recorded instance of Predators going human-hunting resulted in the Pred taking a few trophies and then getting killed by the humans. And these were 20th century humans, without the advantage of 23rd / 24th century technology. Something as simple as a tricorder would have removed a lot of the pred's advantage.
Also, the toughness of the Yautja themselves seemed to vary strangely. By the time of Predators, they seemed to have become less tough than they were previously portrayed. A guy jumped on one from behind and stabbed it with an ordinary knife, another guy with a samurai sword fought one to the death. In the earlier films they were pretty much bulletproof against all pistol rounds fired at them. They took some damage from rifle calibers and shotgun rounds, but it was much less than the same weapons would have inflicted on a human. I'm not sure what explains the discrepancy, other than sloppy writing / film making. Unless one was to argue that the wire mesh body covering actually projected some kind of force field.