The ancient Greeks commented on the rigid hierarchy of the Persian army — and not much has changed:
American and British forces in Afghanistan, for instance, have commented that local troops can be ferocious in combat, and like the action of getting into a fight. (I have this from personal accounts, and military publications.) Their main weakness is in their officers, especially the NCOs. Whereas American NCOs are trained to take initiative, especially when higher organization gets disrupted during the fog of battle, Middle-Eastern officers are wary of doing anything they might be criticized for.** Success as an officer is not necessarily a good thing. Outstanding success makes one a political threat; it also could be interpreted as showing up one’s superiors. Extrapolating backwards to Alexander’s time, there are numerous reasons why ethnic troops and their lower officers would not fight vigorously for their Persian commanders, if the battle started going against them. Generals who failed risked being executed; but generals who succeeded were potential rebels, and many of them got executed or assassinated in a few years anyway, in the distrustful politics of the Empire.
**In this respect, the Roman army was more like the contemporary American one. Centurions — leaders of a company of 100 — were widely regarded as the backbone of the army, and treated as such by successful generals. Also similar were the widespread opportunities for upward mobility in the revolutionary French army at the time of Napoleon.