Mechanical durability simply refers to how dense the pellet is, and how well it is formed. Pellets that are denser are of course stronger, the advantage is the pellets withstand transportation better, and work more efficiently in the pellet burner.
When a quality pellet has exited the pellet press, it should have a smooth surface, with little or no cracks. If the pellet is cracking and expanding it is because there is too much moisture within the pellet, or poor compression within the pellet press. Once a quality pellet has cooled, it should be like a coloring crayon. The surface of the pellet should be smooth, and have a surface shine.
Wood pellets tend to shine more than others; the most important thing is the pellets smooth compact state. Try tapping the pellet against a hard surface, to see if the pellet stays intact, or if they crumble or easily crush and separate. The length of the pellet is not really that important. However if pellets are too long (above 1 inch) they can cause damage to the auger in the pellet burner.