Well, is the unit intended to represent both the workers and a sufficient number of guards to keep them in line? If so, then the negative police value wouldn't make sense.
I think it still makes sense. While you can argue that the guards can keep the workers in line that is not what the unrest represent.
Unrest represents the rest of the "free" population that has to live nearby and watch their kin suffer from slavery as their anger grows about not being able to do anything about it...
It also represent the mind of the guards that has a very unpleasant work and likely are not so happy with what their masters have them do every day either.
Also, as the base ground combat value is increased through research, the negative police value would remain the same, proportionally, which raises the question of why we're giving cutting edge arms to both our troops and our slaves.
Unless the slaves have the same combat values they are not receiving cutting edge arms, they are receiving hand me downs that regular troops no longer needs
![Smiley :)](http://aurora2.pentarch.org/Smileys/classic/smiley.gif)
Still, I agree that there should be some mechanical representation of how forced labor is counterproductive to quelling a restive populace. But isn't that covered by the unrest cost for creating them in the first place? Or is that insufficient?
I think there should be some permanent extra negative cost in policing too. Otherwise a single regular garrison can handle billions of slave workers just given enough time to reduce unrest gradually if they are built slowly.
Of-course you can just make the forced labor unit more expensive in itself (to increase it's maintenance cost and represent the extra policing), but that kind of defeats the point of being able to raise them quickly....