In a recent book from Treadgood that I read he made a very good point that I had seldom considered, I realized the fact from Roman history but never compared the obvious:
The overview was that the Byzantine Armies were extremely effective but only under good leadership while the Roman armies (based upon centurions) did not need elite leadership to be successful.
Now I agree mostly with that, you have major exceptions such as Cannee where Roman leadership destroyed the army , however looking at the Byzantine army you mostly see great leadership win the battles while poor leadership lost they battles.