The fatigue rules introduce an interesting element to the game. Basically, if troops try to do too much continuously, they suffer penalties to fighting ability, defensive ability, and morale. The things that cause fatigue are movement, charging, and morale. To recover to full strength, the unit must spend one full turn not moving.
Swiss and Landsknechte take twice as long to become fatigued, presumably because of the combination of light armor and better training, discipline, and experience. Several other bonuses and exceptions apply to these troops as well. I will describe some of these as we get deeper into the rules, but here's a quick list of what I found flipping through my notes. In addition to taking twice as long to get fatigued, they incur half the penalty of other troops when changing formation. They have the option of forming a true hedgehog if arranged in a square. They receive additional dice in combat, are not slowed by taking prisoners, have morale bonuses that prevent them from breaking, and force morale penalties on anyone they charge.
From what I can see, if there could be an exception to some kind of rule that would make these troops better, they have it. These guys are definitely the supermen of the Chainmail battlefield! I understand that Swiss were considered the best mercenaries at the time and achieved a storied reputation. They did shield the Pope well enough to become his personal guards, but were they really that good?