Hell, less limbs, making lifting much easier.
Besides, I bet the weight capacity on that metal leg is ridiculous. I bet he could one leg press an entire machine filled with 45s with just that one metal leg.
What a joke thinking that its more difficult. 
So you are saying that having a "metal leg" is an improvement in strength and functional ability.
I'm looking at the leg from this picture and trying to figure out where the contractile force is coming from to straighten his leg. It looks to me that just the glutes are contracting on the right side and the metal leg is more of a brace that simply straightens out. So the actually contractile force is primarily the lower back, glutes and his good right leg and the metal leg is just along for the ride and allows the user stabilize his position and remain upright which I imagine requires consider practice until you can comfortably and confidently stabilize yourself and not fall.
It appears that there is a considerable lack of contractile force doing any kind of squat-like movement with one of your legs having no contractile force making you considerably weaker and unstable. I imagine he could be knocked to the ground much easier then when he had two good legs.
