Those who make their living evaluating players are as subject to the same variations in quality that affect any other profession. There are ones who are extremely good at it, and there are others who will probably be trying their hand at something else in a year or two...and everything in between.
The ones who are really good at it don't need to see much to recognize tools and potential. The ones who are really good at it also work hard enough at it that they make a point to get multiple viewings (either their own or from trusted others), and they talk with players' coaches they believe they can trust in order to get as good a fix on their "makeup" and game-day qualities as they can.
A final observation: The parents who seem quickest to single out players who were "over-rated," condemning recruiters and scouts in the process, are the ones whose sons are having the hardest time getting noticed.