Depends upon the design
All convert potential energy to kinetic energy, but the method of conversion varies greatly. But the assumption that simplicity is wrong. They are complex machines and some of the modern designs are some of the most energy efficient engines ever invented (a well designed and maintained falling block trebuchet can be an amazing 66% thermodynamic efficiency).
Gunpowder replaced catapults as seige engines only because they were easier to transport and contained a far greater energy density...but gunpowder never came near the thermodynamic efficiency of a well maintained treb, even the traditional ones.
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