When I see the critique of Mitchell and that his hands are holding him back, my mind jumps immediately to 2 players:
Limas Sweed, who would get ridiculously open and had the looks of a future stud, but could not overcome his inability to catch even easy throws despite getting such phenominal separation, and
James Jones, who also was also able to get substantial separation - especially on long routes down the sideline - but who had hands that betrayed him, but worked so hard in his 3rd offseason catching tons of throws and working with the Juggs machine that he overcame his flaws and turned into a quality WR.
Hopefully Mitchell goes the way of Jones and not that of Sweed.