Another aspect affecting strap failure is the angle at which they are attached relative to the load-path.
I agree that this is an important consideration because the tension in the straps is a non-linear function of the attachment geometry. If I remember my physics correctly, for a restraint attached at three points, strap Tension = Force / (2 * cos (arctan (w/2*d)) where w = the distance between the floor attachment points and d = the perpendicular distance from the line between the floor attachment points and the cargo loop-back (tie-down) point.
So When "d" becomes very small with respect to "w", very small changes in Force (the force produced as acceleration acts on the restrained mass) can result in extremely large changes in the strap tension. Either the strap or the floor anchor points could fail and it might only take one to start a domino effect that causes a complete failure.