I don't understand people who complain Byzantium is underpowered. If anything, it's OP.
People say it's impossible to ally Poland; I've been doing it all the time in 1.13 provided I have a spare diplomatic advisor from the start (them rivaling the Ottomans, which they typically do, helps but is not necessary, nor are gifts).
But the thing is, you don't even need to ally Poland - sometimes Hungary is enough, sometimes Aragon is enough, as long as the Ottomans are not allied with both Crimea and Tunis (in which case the only way to win is to ally both Hungary and Aragon, which respectively neutralize Crimea and Tunis). If for some reason the Ottomans bring no allies to war, you may fight them alone with the old ships wall trick.
Sure, it's luck-based - sometimes the Ottomans crush you no matter what, sometimes they get embroiled in wars against a lot of western or eastern powers and they lose land without player intervention (once Macedonia and Epirus were released to me as the result of a war lost by the Ottomans against the Mamluks and other muslims) but such should be the life of any minor. The thing is, winning as Byzantium is definitely doable.
If there is any nation that is underpowered, these are Castile/Spain and Muscovy/Russia, while Poland and Byzantium are OP.
And no, do not come to me with Greek Fire, Theodosian Walls and other nonsense. Sure, I would like to see the country being called "Romania" or "Roman Empire", I do that in my personal mod, but it cannot come like that in Vanilla. The Purple Phoenix DLC, which I enjoy and play all the time, is already ahistorical enough. No more than that.