>The decision on whether one's work is "correct" and worthy of a phD is so subjective, and prevailing theoretical trends are so arbitrary and subject to change, that it's easy to reject a dissertation based on cliquish whims.

The other half of the double-whammy is there are a lot of humanities degrees with no marketplace value beyond "has a degree" unless you have a PhD, so anybody in the position to impede your doctorate has more power than he would in a STEM field.