Surely one of the biggest differences between practicing UAs in the Sim as opposed to a real (light) aircraft is the lack of (and/or unreal) physiological sensations in the sim?
For UA training to be effective you also have to add in the physiological sensations. It's all very well being able to interpret the instruments but it's having the experience and discipline to do so when everything else that you feel is telling you something different - hence all LOCs happening when IMC or at night.