I think Baldy summed it up nicely. Coleman hasn't played WR that long and is still learning "nuances" of the game. He's a bit of a project and needs time to learn.
He can't separate at 4.61 so he'll have a CB or safety in his hip pocket.
But Bills look to throw him jump balls where he can go up and out leap the smaller defensive backs using his basketball skills to box out. That's how FSU used him.
A better Gabe is what it looks like.