Actually those are two extremely different situation. The main issue is in the ability of slaves to integrate into regular society. For example in rome there were pretty damn good provisions for slaves to integrate into society once they became free men. And a key difference between the majority of Roman slaves and Americans slaves is once a Roman slave became a free man unless they explicitly told you they used to be a slave you had zero clue they were.
The cultural attitude to behind slavery and which type of people were enslaved in America was totally 100% different. In Rome the attitude was more of a typical right of the conquer to enslave the conquered, but if you were free you were free. The blatant discrimination against non-Roman ethnic persons while still existent to some extant is in no way comparable to the treatment of black slaves in America or black free men. A black slave becoming a free person in ancient Rome would still have gotten a little shit, but nowhere near the amount that a black person in America would have. I mean hell nearly 100 years after the end of slavery we had to send the national guard to ensure that black children could attend the same school as white children and a white's only pool was drained because a black woman touched the water. Even certain programs like the GI bill and federal housing loans were biased towards white people well after the ending of slavery. The issue isn't just slavery, but the social attitude used to justify slavery in the first place which didn't magically disappear after the civil war ended.
Learn your history. Yes every race has had some sort of system of slavery. But that doesn't mean that the institutions were even remotely identical.