Imaro
Legend
My 2 cents is that it isn't any harder, it just has a different focus. In BBEG campaigns (and stories), there is usually a pre-set goal of defeating the BBEG...this goal exists independently of the PC's and is usually the driving force for the majority if not all of their actions. S&S fiction however seems to deal with personal goals and thus a long running campaign is going to need PC's who have long-term goals or evolving goals who are willing/forced/driven to act in order to achieve them. I don't think power is especially relevant since S&S fiction in the vein of the Eternal Champion stories and Karl Wagner's Kane stories deal with some pretty heavy hitters as protagonists (immortal sorcerers, god-summoning proto-humans, etc.) but they don't do what they do because they are motivated to save the world, they do what they do because it suits whatever goals they have and that's what I think is important, whether it's just becoming richer, winning a kingdom, giving meaning to an eternal existence or even destroying your homeland... It's their goals that give them purpose and create the S&S heroes stories.