McKenzie obviously has return talent, but he's just more boom-or-bust than Roberts. Roberts was like the Frank Gore of return men. Always gonna get positive yards and make some big plays, but probably never take it to the house.
McKenzie will lose yards sometimes because he'll run in circles. But we saw him take it to the house against Miami last year, I think our only return TD, despite his limited touches.
The positive thing is that the Bills offense is now good enough to make up for the worse field position that may result from a McKenzie mistake.