As to the exams...I don't know. I took parts 1 and 2 of the national boards in med school and studied and did ok I guess. Part 3 we all took about halfway through internship, where there is no chance to study, and after taking call the night before. I passed. My buddy, who is now a vice chairman of a major academic dept sat down, filled out all the answers as "c" and was done in 3 minutes. I guess he passed, and again now is a vice chairman of a surgery dept here in the US, so I'm guessing part 3 is not to challenging!
Specialty board certification exams on the other hand are pretty thorough. In my specialty of Otolaryngology, you take a written exam in the fall, and if you pass you take an all day oral exam in the spring. But its on stuff you actually need to know in order to practice...so its a good test and if you are prepared, you pass.