It can be more difficult if the breaks go against you instead of always going in your favor. Strong neighbors can decide to attack you when you're least ready for it, instead of remaining peaceful. Defeated factions can re-emerge in rebel-held territories on your border with huge armies and immediately attack you before you can muster enough to oppose them. Exommunications can prove somewhat difficult to deal with, costing you international friendships and the loyalty of your own people. You could find yourself facing many enemies on multiple fronts, just barely holding the line.

Or, none of those things could happen, as you've seen. The AI can cooperate nicely with your plans for conquest. You only have one enemy at a time, defeated factions stay down, and the pope has too many of his own problems to think about you. With this game, you never really know what's going to happen next.