The big problem with Byz in early is that most Byz units require a pretty good tech level. That limits unit production to Constantinople except for HA and maybe Byz Lancers. Also, the Turks early unit roster is pretty powerful in the hands of the right general. They also can produce most early units in most provinces fairly quickly. Add in Jihads, Syrian assassins, and AHC within the first 20 turns. The Byz also have to deal with Hungarian Szelskey if you are playing VI.