I don't think end game will be much of an issue as there will be a level cap. That means the final encounter can be tailored for a party that is level 20 or whatever the cap may be. But the battles in between could play out much differently depending on how a player goes through the game.
From what we've been told there doesn't seem to be a hardcap on experience or levels, just a point in which you will run out of things to earn experience from (in other words, there is a finite amount of XP available). Of course, the problem there is that one player may consume 90% of it before reaching the end of the story while another player only gathers 60% by focusing solely on the main plot, and the designers may have designed the final encounters around the assumption that players will have accumulated 75% of the XP available.
But, yes, if there were a hardcap it would be easier to predict/control how far everyone has advanced by the end.
