Welp, spent 20min writing a response to a post that was deleted. But I'll keep the last bit which pertains to this question.

The biggest difference between ancient slavery and our concept of 'modern' slavery is race. Slavery is so heavily tied to race in our modern world but this was absent from the ancient world. Slaves could be of any colour and from any locale - they could even be Roman. Piracy was a big problem for many periods and captured merchants and passengers could easily be sold off as slaves. If such a slave was then sold to a Roman household, why would they believe he was Roman? Any slave could make that claim. Then you have foundlings who were abandoned by their parents (for whatever reason) and then raised by a slaver to be sold. So it was impossible to have a 'slave race' as they could be Gauls, Greeks, Italians, Numidians, Persians, Syrians, Egyptians, and even Romans. And even the name given to a slave means nothing about their origin. A slave with a Greek name could be actually from Greece, or he was sold in a Greek city (there were large slave markets in the Greek east), or the slave trader who named him was Greek.

Furthermore Roman society was quite remarkable in that they integrated their ex-slaves into Roman society. Generally city-states tended to guard their citizenship quite zealously but any freedman became a citizen (albeit with some restrictions). One generation later they were not even tarred by the status of freedman anymore, they were fully Roman. While the elites would sniff at freedmen and those of freedmen origin, we don't see any sign that ordinary Romans had an issue with them (since they may have descended from slaves as well). Freedmen were quite proud of their new status would demonstrate that often with monumenta. Data on the number of freed slaves is very fuzzy (along with the numbers of actual slaves) but some data from Herculaneum suggest that 2/3 of that population had servile origin. We ultimately cannot get firm numbers but can only say that being a freedman/servile origins was a commonly shared experience among Romans.

Now as to the treatment of slaves, I suspect it had similarities with more modern examples (although I freely admit no expertise in the latter aside from general knowledge). Most slaves would have been used for agricultural work and would have been treated harshly, kept in line through pain or fear of the infliction of pain. It would be a hard life of endless labour, but at least one without racism adding to the suck. Manumission would have been incredibly rare for these unfortunate individuals.

Urban or household slaves would likely fare much better. They'd have regular contact with their master and the opportunity to build a relationship and thus receive better treatment. For these slaves, manumission would be dangled as a carrot to encourage 'good slave' behaviour - although that prize would likely be given to very trusted slaves (e.g. Cicero's Tiro) or those with economic potential (e.g. trained craftsmen). On the downside, contact with the master would open them up to rape - which must have been a common experience for all slaves regardless of gender or age.