I am pretty sure it will bend after a fall for an horse galloping.
Rolling as in judo rolling, to absorb the blow by curving yourself and being as relaxed as you can be (not sure this sound right in english. oh whatever)
Metal armor are not good at absorbing blows. that's why you have something like layers of silk (silk, the medieval kevlar...).
So, IMO, you have a lot more chances to break your neck or limbs in full armor than in clothes. More weight, more pressure, more inertia. Oh, and you will absolutly one-shot the poor guy you are falling on (poetic vengeance)
But no, I will not test that Myth Buster-like.