The thing that a sim never can simulate (not even a full-motion sim) are the various ways the vestibular system can get confused by e.g. the coriolis force when

The thing that a sim never can simulate (not even a full-motion sim) are the various ways the vestibular system can get confused by e.g. the coriolis force when you move your head while in a turn that makes your feel like you're turning around another axis. Most sim pilots can fly on instruments without a problem, because it's precisely the conflicting information between the instruments and your vestibular system that makes it easy to get disoriented. Flying a sim, you'll never have your ear telling you that you're leaning on your side even though you're flying straight and level, or vice versa, so you'll never be tempted to side with your vestibular system. In a real plane, just turning a tight turn and suddenly looking down on the floor because you dropped something can induce conflicting vestibular information that's quite disconcerting even in day VFR.