I would struggle to draw meaningful comparisons between Antebellum American slavery and "ancient" slavery primarily due to two factors: the vast range of time we're speaking on and the varied but widespread implementation of slavery over that time period. Slavery in West Africa vs Europe vs the Middle East vs China already ranges from working off a temporary debt as a relative freeman to helpless human sheep to a religious slaughter, and that's without considering how those institutions evolved over thousands of years.
To give a very broad(but fair) answer though, the answer as suggested would be: your mileage may(wildly) vary. In the Antebellum American South(as well as a few northern States) slaves could not testify in court in cases involving whites(https://scholarship.law.missouri.edu/cgi/viewcontent.cgi?article=1954&context=mlr pg.4) , which was more or less the same for slaves in Ancient Rome unless they were tortured en masse, and in general most slave societies except Muscovy and Assyria(https://www.britannica.com/topic/slavery-sociology/Family-and-property). Likewise, in Antebellum slavery, raping a slave was often legal(if socially condemned) which as a general rule was true for most implementations of slavery, but in others it was illegal to rape women or married women specifically.
There's many more examples to touch on, for instance castration in American slavery was a possible punishment for slaves who misbehaved(http://crab.rutgers.edu/~glasker/African-AmericanLectureSlaveryinNorthCarolina.html), for a large portion of Chinese and Ottoman history, people were deliberately castrated then cast into roles as slaves and/or administrators as eunuchs, sometimes for real crimes, some times for bogus crimes or reprisals, and sometimes just brought in from abroad(for the Ottomans specifically). Other types of slavery were contingent on the enslaved not being of your avowed religion(Muslims enslaving Christians/Jews and vice versa http://ldhi.library.cofc.edu/exhibits/show/african_laborers_for_a_new_emp/slavery_in_iberia_before_the_t).
Overall though aside from throwing examples at you, I wanted to take the time to get this out because this is a particularly controversial topic and I didn't want someone blundering in saying "oh nothing compares to how bad it was in the American South" nor would "slavery's been around forever and people need to stop hyperfocusing on the south" be factual or appropriate. American Antebellum slavery was pretty darn ugly, there were "nicer" forms(pawnage) throughout history, and there were decidedly "worse" forms as well(human sacrifice). If I absolutely had to put them on a scale of 1-10 with 1 being the absolute worst I'd probably put American slavery at a 3. But really the important thing to bear in mind is that they were all awful.