I tried playing as Byzantium and had to restart multiple times to get the alliances I needed to survive. The requirement to do so based on RNG is silly. If all possible allies that can help deter the ottomans are either rivaling you or allied to someone rivaling you, you are screwed.
Other than that, the main thing I noticed was how...lackluster restoring the empire was. Yea, theres a lot of fighting, but the rest of the world has no real reaction to it, there are no events and the missions are basically "okay here you go, you get more claims for completing this mission". You dont get anything to actually help you complete the missions.
For example, Great Britian gets stuff like naval boosts to help them obtain naval supremacy, a conquistador with 80 tradition and stuff that develops their provinces. Everything is flavourful and fits the missions well. Byzantium gets almost nothing, they get no naval boosts to help them recover the shoreline from the Ottomans for example, and the progress of the claims is odd.
Recover eastern islands is a top level mission, and it involves recovering the Aegean islands from Venice, Naxos and Genoa. But you dont actually get claims on these islands! You do not start with the claims and the mission doesnt give you claims...completing it just gives you claims on Malta, etc, which is skipping ahead imho. Infact, Byzantium should be starting with cores on everything they lost in the last 150 years which is when cores expire in the game. The idea that you have to fabricate claims to recover provinces you used to own is silly, especially when later missions in the tree give you the claims you need. Not even starting with a claim on Naxos is particularly strange.
It doesnt make sense for recovering the coastline to be a top level mission either, because that requires obtaining naval supremacy and launching an amphibious invasion against the ottomans, and this should NOT be your first mission...the logical progression is to kick the Ottomans out of Greece, Bulgaria, etc, and after securing the western half of the empire, you start thinking about going into Anatolia. Recovering Bulgaria should give you naval bonuses that you would use to take on the Ottoman fleet, which would make sense progression wise. Instead, the missions encourage you to invade Anatolia via the sea while leaving Bulgaria alone...which makes no sense whatsoever.
And a huge problem with restoring the empire is all the cultures you need to promote. Yes, going humanist helps, but they could at least throw Byzantium a bone and give them more culture slots...the western half of the empire alone has the following as non-accepted cultures :
-Albanian
-Serbian
-Croatian
-Bulgarian
And of course, going into Anatolia requires Turkish. Then the middle east has a ton of cultures, and Italy alone has like 4+ different Italian cultures...the diplomatic cost to convert the bare minimum amount of cultures is astronomical. Byzantium really needs more culture slots from missions to pull this off properly. It wouldnt even be OP at all considering other nations get WAY bigger bonuses for their missions.
Its quite clear that not a lot of thought was put into this DLC...