[SPOILERS]Question about Luffy

We all know Luffy has Haoshoku Haki, however I'm alittle confused on the magnitude of this ability. We have seen Luffy knock out 50,000 fishmen pirates with it in one go. After witnessing the "Battle at Marine Ford", I wondered why Whitebeard or Sengoku ever used it to wipe out their opponent's army. The only valid reason I can come up with for this is that Luffy is the only one capable of displaying Haoshoku Haki of this magnitude. I believe Shanks is also exceptionally skilled with it since he's one of the only other characters (other than Silvers Rayleigh) we've seen to be really good at it.

So what do you guys think? Is Luffy just really bad ass, or is it just coincidence that the big leaguer's don't often use it?

Another question I've had on my mind was whether or not it causes fatigue to it's user. I doubted that it does though since Luffy wasn't tired after wiping out the fishmen.