Well I dont know about if Byz will be invincible but of course val 1 varang will be a strong unit add pronois and alans for cav and yes they look strong.
A simple solution could be to say that each player in the same team must use diffrent factions. I.E. a team of four players must use 4 diffrent factions.
Almohads are a bigger problem though, with val 1 they cant even reach 8000 if not going way passed the 4 unit of same sort and russians might have the same problem. Egypt and turks on the other hand I think can field very good armies as always :)
Kalle