The Nomadic Military Advantage: why were nomads so difficult to fight?

SOURCES:
The Perilous Frontier: Nomadic Empires and China 221 BC to 1757 AD, Barfield
Ancient China and its Enemies: The Rise of Nomadic Power in East Asian History, Di Cosmo
The Scythians: Nomad Warriors of the Steppe, Cunliffe
Masters of the Steppe: the Impact of the Scythians and Later Nomad Societies of Eurasia, Pankova et al
Nomadism, Evolution, and World-Systems: Pastoral Societies in Theories of Historical Development, Kradin
Structure of Power in Nomadic Empires of Inner Asia, Kradin
Empires of the Silk Road: A History of Centra Eurasia from the Bronze Age to the Present, Beckwith