a conquistador named bernal diaz del castillo wrote about the maquahuitl in his account of cortés' conquest of mexico, "The Conquest of New Spain":

"While we were at grips with this great army and their dreadful broadswords [maquahuitl], many of the most powerful among the enemy seem to have decided to capture a horse. They began with a furious attack, and laid hands on a good mare well trained both for sport and battle. Her rider, Pedro de Moron, was a fine horseman; and as he charged with three other horsemen into the enemy ranks--they had been instructed to charge together for mutual support--some of them seized his lance so he could not use it, and others slashed at him with their broadswords [maquahuitl], wounding him severely. Then they slashed at his mare, cutting her head at the neck so that it only hung by the skin. The mare fell dead, and if his mounted comrades had not come to Moron's rescue, he would probably have been killed also."

so apparently these weapons could decapitate a horse, much less a man. volcanic glass is some scary stuff. in a trip to hawaii for geology back in my college days, we had to wear special boots just to walk out on the old lava flows, and you had to go through several pairs of those boots in a field season.

i imagine most armor would have been extremely effective against the weapons, though.. glasses tend to break rather easily. but maybe they had ways of tempering it somehow?