The problem with the First Council of Nicaea was that it was decided wrong. The whole "there are three gods, but only one god" is inherently confusing. There's a reason why Arianism keeps recurring over and over again. All the new nations who have been introduced to this aspect of Christianity find it bizarre.
If the decision would have been more along the lines of Islam (i.e. Jesus is super holy, but not God) then it would have been easier to maintain unity. In fact, Islam's adoption of a form of Arianism is one of the reasons it replaced Christianity so quickly in North Africa and the Middle East. (Well, that and the sword.)