I have found, from a training deficiency standpoint, that the vast majority of flight school returnees do not have a FAA instrument-rated level of knowledge despite the fact they return with commercial/instrument tickets. Most can not accurately decipher an approach plate or enroute chart. Most do not know the elements of an ATC clearance, much less communicate effectively with ATC. Most can't remember rote items such as when an alternate is needed and the rules for selecting an alternate. I'm guessing the reason they lack this knowledge is the span of time between Instruments and their advanced aircraft qualification though. (i.e. Reasons people forget - Disuse.)