Minecraft wiki entry for Things_not_to_do, associated text: The player should avoid bringing highly enchanted pickaxes with them. The Efficiency enchantment is useful for breaking blocks quickly, but using it in the Nether when mining netherrack may lead to death. Netherrack is a soft block and high efficiency is not needed, because you don't want to mine netherrack so fast by holding the mine button that you drill out large quantities of netherrack quite quickly and dig yourself into lava pools or rivers. Also, netherrack has limited use and could end up wasting durability of the player's Efficiency pickaxe.