Just keep in mind, that there is a big difference in autorotation between a High inertia rotor system and a low inertia rotor system (i.g. Bell 205 vs R22).
While you still have lots of "power" available with low rpm in one system, you will find you´re running out of power in the other system-BEFORE the skids touch the ground.....(even in "normal loading conditions" without heavy weights)....
What can happen in an overspeed?
Main rotor head and transmission can shear, blades could delaminate-when you´re operating outside the given parameters of the flight manual.....