To answer the original question, think of it this way.
What cut of meat gets the most question for help. Having run a forum for 10 years, hands down I've seen 10 questions about brisket to any other cut.
On AVERAGE, it just appears to be a difficult cut because if you don't have good Q knowledge (or access to this forum) and you don't know heat control, Brisket is the cut that can really define how good you are at the basics.
2nd toughest might be Ribs.
PB is pretty damn easy
Chicken, if people didn't ask questions about chicken skin we probably wouldn't have any posts about chicken.
Brisket is the toughest because it exposes your weaknesses as a cook.