Even if both nations are voluntary forces the south has to spend less time training them since many of those patrols get military like training and gear. Slave rebels and partisans are basically live target practice to confederates. Those partisans also have guns and know how to use them(confederacy is learning how to fight partisans. Think how that relates to changes in war in the next 100 years).
If the Confederacy is dealing with organized partisans they will have to develop the tactics to do that, they will need to use actual Confederate soldiers, and they will need to use significant forces. If you try to use militias and slave patrols for this difficult and very dangerous job, the possible outcomes are:
* The militia charges in and get wiped out.
* The militia refuses to take on organized partisans.
* The militia spends a few days "looking for" Unionists in places where they don't think there are any Unionists, then report the Unionists have escaped.
* The militia spends a few days "looking for" Unionists in places where they don't think there are any Unionists, then report there are no Unionists.
* The militia ambushes and kills a couple of random poor white men that they don't like, steals anything that isn't nailed down, sets fire to their homes, and possibly rapes their women.
* The militia ambushes and kills a couple of random black men, steals anything that isn't nailed down, sets fire to their homes, and possibly rapes their women.
After enough of these failures, the Confederacy probably will learn the proper methods for their regular troops to beat Unionist partisans. This will prove useless if they have to fight the US, which will have repeating rifles, Gatling guns, and barbed wire. It would be less useless if the Confederates decide to act as partisans against the US troops, but they will regularly be betrayed by the slaves and surviving relatives of white Unionists.