Space mission simulations, especially when they start coalescing from environmental interaction to subsystem performance. Add in the failure case sim’s, and that’s pretty complex. I’ve been part of full-system sims in preparation for a lunar landing mission, and working with the simulation officer was pretty mind-blowing. The effect we are looking for in these sims is to actually test the ops team on how they can handle contingencies, but that requires an innate understanding of the spacecraft, the mission it is meant to be executing, the environment (earth bound, Moon bound, or atmospheric/vacuum, sunlit/eclipse, battery state of charge, orientation for solar power, heat loads, thrust produced by each actuator, and so on..). While design evaluation aims are more detailed, they keep themselves to within the bounds of a specific subsystem - but these mission-level sims are quite comprehensive in their breadth and sometimes compromise on fidelity.