The abrasive powder destroys the waxy layer and the cockroaches die of dehydration within days. Exoskeleton can also be festooned with spiky poky bits to make them a mouthful. An arthropod's armour is segmented so it is flexible. However, their joints are not like our ball and socket joints, so these joints can only move in one plane. Having many segments or narrow joints offsets this limitation.
Many predatory wasps can carry prey bigger than themselves.
Humans can carry only about three-quarters of their body weight. Although to some extent, having muscles inside an external skeleton is more efficient use of muscles, arthropods are strong mainly because they are small. Their muscles exert the same force as our muscles. For large animals like us, the ratio of the muscle volume to surface area is much higher so our muscles are relatively weaker. Unfortunately, exoskeleton being tough, can't expand.