Yeah, an obsidian arrowhoead would cause nasty lacerations if it managed to hit unarmored flesh; if it pierced far enough, it might even strike a bone and shatter. Obsidian weapons, by their very nature, will absolutely mangle skin and muscle with disturbing ease--even more so than the finest steel swords---, but as was said earlier, they're rather brittle and incredibly ineffective against armor. 

A macuahuitl will cleave through the neck of a horse. Confirmed on a documentary showcasing the weapon's power, though the wielder was not the largest man in the world and he was attacking what appeared to be a slab of raw (if not spoiled) slab of beef. He did manage to cut about 6-8 inches into the meat (using a side-ways swing), however. 

One wonders what would happen if a stronger and trained warrior with sufficient downward swing would do to an unarmored person, and what would happen if the weapon was augmented by applying more weight to it (perhaps applying stones tied along the length of the 'bat').

I think two-handed versions of the macuahuitl existed as well.