All of the above are true but I will try to expand on it a little more. The matchup zone coverage usually puts them in man coverage on the slot receiver. Simple enough. However, they are required to play the run like a linebacker. So now they have to cover while diagnosing long enough to determine whether it is a run or not. This would be hard enough for any db but it gets even worse when you are trying to do it against a receiver that is generally most effective on short, explosive routes.
The risk is that you need someone with:
True DB coverage skills
Block shedding and tackling on par with LBs
Can read the offense pre-snap(at least run or pass, not necessarily the exact play) and be correct >90% of the time. If they get it wrong you may as well have a punter covering the slot. Missing on a run(thinking it was a pass) means the slot is already running the nickel out of the play and now he has to completely changing momentum, fighting through blockers that have a size and angle advantage, and trying to help in the run fit. Most people will lose this scenario almost every time. Missing on a pass(thinking it is a run) means trying to hold up the slot long enough to diagnose run, hoping the jam doesn't let them run completely free, then trying to chase them down from behind before the QB gets the ball there. Most people will lose this scenario almost every time as well.
Someone with any one of these skill is probably pretty high on the depth chart at their position. The tricky part is that all of this has to exist in the same body. The time it takes in the film room to get to that pre-snap read level seems to have been the main thing holding Mr. Jackson back.
The payoff is that your LBs can focus more on the run and your DBs can focus more on the pass at the beginning of the play. Not requiring your other players to do as much(though their jobs are nowhere near simple!) means they can cheat, even if just a little bit. When everyone is doing their job well the defense looks faster because they actually are half a step quicker to their responsibility.