1/ One major thing they don't tell us in FCD is how fast a player will develop. It's a hidden rating. Two similar 18 year olds may be different in their development patterns, one may become a stud, the other not. But they have to get ample playing time to develop to full potential. If you sign an 18 year old and sit him on the bench, he'll go up one or two points anyway. If he plays 30 games he'll go up six or seven.
2/ Shooting accuracy. The other ratings are good for making them better overall players, ahd helping the rest of your team. Certain ratings like speed and offensive instinct are good but you can have effective forwards who are slow, or with low instinct. But a forward with low accuracy will, no matter what, take a lot of shots that aren't shots on goal. Those are blown scoring chances.