Here's where even the golem example breaks down: don't you think an obsidian golem has a shatterpoint, or that a stone golem might develop a crack? If you were watching a movie, and someone lodged a crossbow bolt into a metal golem's neck joint, would you say, "No way, golems can't be critted?"
As to the 6 on a 1d6 vs crit question... hitting the skeleton on the neck is a crit. If there is such a thing as a crit, that's it, because that skeleton is going to be in bad shape. Even if it's a higher HD skeleton, a lucky blow like that would wreck it. Similarly, what if you manage to hit a skeleton square in the chest with a mace, breaking all of its ribs at the same time? Crit.
As written constructs and undead have no weak points. Do I believe constructs and undead have no weak points? I do not. Maybe a monster that was a superdense sphere of flexible metal with no limbs.