Southern defenses of slavery were accurate in legal terms, possibly accurate economically, but not defensible morally.
A major Southern defense of slavery was that it was a more humane system than the Northern wage labor system. There is no legal aspect to this argument (but since slavery was without doubt legal I say their defense was accurate in that sense). Economically, this may have been true, though it is not easy to tell. Northern factory owners did not take care of wage laborers when they were sick or old, for example. They did not pay them very much either. However, there is no moral equivalency here. Wage laborers were at least free to leave if they could find other work. They were not subject to having their wives raped or their children sold. Thus, there is no real moral equivalency.