Metcalf's physical talents were apparent at the draft combine. As I recall, the concerns were why those other-worldly skills had not really translated to a dominant college career. You have to decide what data to trust, and apparently NE relied more on game results that, while not bad, were not as awe-inspiring as one might expect for such a physical beast.
The draft will always be an inexact science because you are juggling how to weigh what you see on the stopwatch, the game tape, the coach's evaluation, your interviews with the guy & his personality /motivation/ love for the game that you have to determine based on a tiny snapshot.
It was a miss, by many teams, including NE. To his credit, Metcalf has been motivated by his draft slide to work more on his craft.