I practice Full Contact Karate Ashihara (from Kyokushin) Karate and I wouldn't block a FULL force kick with a standing Gedan Barai
For me its simple:
Arms for Arms, Legs for Legs
This means block low Kicks (below the waist) with your shins/Legs (below the Waist) and punches (above the waist) etc with your arms (above the waist).
The medical explaination is:
The Tibia against the Ulna (Smallest Bone in the lower Arm) the Tibia (biggest bone in the lower leg) The Ulna is one of the most commonly broken bones in the arm and to try and stop a kick with it would do nothing more than break it.
Of course there is some differences but
A man is but the product of his thoughts what he thinks, he becomes.