To be honest, I think the "freezing" mechanic is a lot more realistic. Depending on what mechanisms the robots use to move their joints, and the location hit, stiffness would be the more accurate result. If, say, their joints move with mechanical force caused by gears, partially destroying one of the gears (and likely causing it to melt, seeing as hit voxels glow) would likely clog the system, which would resist movement. On the other hand, if a joint were powered by, say, hydraulics, severing the line/piston would cause it to go limp.
That being said, in my opinion, ragdolling seems to be more suited for organic foes, while freezing seems appropriate for mechanical enemies.