American slavery was not that bad in general, but the propaganda is strong. They generally weren't beaten that often, they were allowed to have families, they had weekends off, they usually got sick days, they often got educated, and they basically only got killed or mutilated if they did something seriously bad. They had lifespans similar to free-born farmers, too.
The only comparison for how well those slaves were treated is Greek/Roman city slaves, who had cushier jobs, but slaves headed for the fields and the mines in that era had lifespans measured in months. Slavery in the East (Arab countries and China) was brutal by comparison.
People forget how shitty life was in the 1800s, and they use the juxtaposition of slaves (who would otherwise be lower-class peons) and plantation owners (rich landowners) to pretend that American slavery was so brutal. Free farm workers didn't generally have better lives than slaves, but it was just an economic calculation about whether it was better to own the workers or employ them. By the 1860's, there was no contest: it was much cheaper to employ them.