Accu-Sim is our own independent engine that we've been building over recent years, that is interfaced with FSX. Little by little, we ask for less of FSX. Accu-Sim now controls the entire guts of the aircraft beneath the skin (engine, prop, all systems, gauges, sounds, etc.). We use FSX for its excellent open-architecture engineering. Presently, FSX is closer to being just a 3d engine and environment for us but it is still far and away the platform of choice for this purpose.
And yes, no question, BOB2 WoV is a jewel as what it does do, it does better than any other.