| Yep, that's the big part of it. Zerg can play in many different ways and yet is always vulnerable to some near hard counters from P or T. They really need to control the game to do well and capitilize off mistakes so very effectively.
A very good Zerg though can make even a good opponent look bad just by forcing him to play reactively from first contact. The tech switches and sheer number of possible and effective zerg builds essentially force the opponent to maintain scouting or start flipping coins. I guess that's why I like Protoss v the top Zerg players though as they can, with some heavy effort at times, maintain the scouting needed to counter.
Hell, they are all potentially good matchups though. | 