Well it's obvious the Dunyain have weaknesses, they are still humans.
From the start Khellus has been proven wrong and made mistakes. Do you remember Leweth the trapper with his dogs ? How Khellus described him as lost in his superstitions, seeing the mystic everywhere ? How pathetic the guy seemed....
For Leweth, Kellhus knew, the world was fraught with gods, ghosts, even demons. It was steeped in their conspiracies, crowded with omens and portents of their capricious humours. Like a second horizon, their designs encompassed the struggles of men—shrouded, cruel, and in the end, always fatal.
And now after Khellus has travelled the world and lived outside the Dunyain's refuge, he has accepted a lot of what he thought was myths and superstitions, he's become much like what he despised in the 1st world born he met. Let's face it if there is one glaring weakness in Khellus and probably the Dunyain as a whole, it is arrogance... Being smarter doesn't mean being wiser or more knowledgeable.
One thing that surprises me is how the Dunyain are described as a sect seeking enlightment, and exiling themselves willingly from the world. But then there's the whole training about face reading... For me that's a tool for manipulation and control, I can't accept that's it's needed for suppressing your own emotions. So the Dunyain may have exiled themselves but they are definitely not above world matters.