Well, as co-designer of Dark Sun Online II:The Age of Heroes (a project sadly never completed) let me give you a few of the ways we balanced things:
(1) Sorcerer Kings are ultimately competitors. And totally self-centered, so they act as checks on each other pretty well. Noone wants any other one to get too more powerful then they are. With that to worry about, you don't need the hassles of a peasantry in active revolt too. Any weakness will be lept on by your 'fellows'.
(2) Remember that Dark Sun is fundamentally an eco-fable. Sorcerer Kings are Defilers. The Druids and their preserver allies represent an important counter-power trying to return the world to a natural state, which means limiting and eventually eliminating the defilers.
As the representatives of the fundamental forces of the world, what could be considered "true gods', they are a real danger to the SKs.