For the record, I´m in Brazil, train BJJ too, and can tell you that the first two things we learn how to do here is avoid getting kneed or punched severely in the face when entering an attack, and the armlock. Along that line... if the attack doesn´t connect causing at least minor head trauma... your going down!
The other thing we always forget is that it's damn hard to cause head trauma. We get a little too comfortable in the dojo and think our punches/atemi will always connect/work. Anyone watch boxers or kickboxing? Anyone practice kneeing people on a regular basis? Hell, how many of your really practice punching on a regular basis?
The other thing I think I've seen with BJJ is that they don't just drop and shoot. They wait until you are off balance or do things to get you off balance and then they shoot.
Hey, chrisinbrasil or Larry do they do any multi-attacher stuff? My first instructor was adamant about not going to the ground. He'd witnessed one too many bar fights and his take was that once on the ground you were a good opportunity for the spectators to join in with a kick or 2. I can't speak to that but my suspicion is that they must have a stand up component to the practice for multiple attackers that we don't see on the outside.