Orcs are strong in open battles, but they find it more difficult to assault or defend a castle compared to magical races. They have little chance against dark magic. A good example is the city of Zogsox in the campaign for Arantir; attacking it is almost impossible, but it can be taken with dark magic even without significant losses. Magic is a practically universal tool, applicable against any race, while orcs need to be prepared to counter specific types of magic. It's good if you know what kind of magic you're fighting against, but if not... Nevertheless, I really enjoyed playing as orcs; it's pure fun.