I tried to find the site I was reading when I was first considering X-Plane because it spoke about the difference between modeling FDE's in P3D/FSX and the system used in X-Plane. It was suggested that X-Plane's system can model real physics/airflow over the aircraft to model a significantly better aircraft than the "other Simulator games". That's is something they are quite proud of and while they say FSX and P3D are good, they will also refer to them as games. . .where X-Plane is a true Simulator.
As for Subsystems, I've never read anything on their site that speaks directly to that (I assume you're asking about fuel, engine, hydraulics, and so on). That may be something that someone else can speak to, especially someone like the developer you mention.
