It's both. Anything that you effectively have optimized to the details of its environment will end up dependent on that environment. If that environment is the real world, then it's fine. If that environment is a simulator, now you have sim-to-real problems.

There's always going to be a gap between reality and simulation, because simulation fidelity has an incredibly long tail. "Digital twins" are relatives, not identical replicas, because everything about capturing the real world for simulation involves simplification and discretization and abstraction.