Lucky isn't supposed to be a bruiser who plows ahead whether there's a hole or not. Part of running behind a zone blocking scheme is that you have to be patient and look for a crease or a cut-back. If the play call is a straight up ISO or dive Lucky hits the hole as hard as anyone.
It appears that Callahan wants a two back system where one player is more finesse and patience and the other is a bruiser. Lucky is the finesse, don't bag on him because he isn't the bruiser.
IMHO, when you look at the total contribution of Lucky to the offense it's on par with what someone should expect from a top tier recruit. He blocks, catches and runs the ball well and began playing a major role in the offense his second year on the team. I'm not saying he's a superstar, but how many 5* players turn out that way? But people don't look at his total contribution, they look at the box score for his rushing yards and use buzz words like "goes down too easy". For whatever reason people expect him to break 5 tackles and run for 50 yards every carry. No one does that.
I think part of the reason that he's not tearing up opponents like Darren McFadden or Steve Slaton is because he's not the team's identity. Arkansas structures their entire offense around McFadden. WV structures their offense around getting Slaton and White out in space. For those two teams, their entire MO is to get thier marquee players the ball as much as possible.
Nebraska's system is about balance. It's not about a feature running back, or even smashmouth football. Regardless of who the RB is, he's not the focal point of the offense, so of course his numbers won't be spectacular. For whatever reason it's easy for people to see this when it comes to quarterbacks, but with running backs they're blind.