Many scholars argue that part of what made Rome great was its relatively lenient policies regarding manumission, that is, going from slave to freedman (whose children would then be full citizens) was a great motivator for slaves to be quiet, content, and productive. R.H. Barrow in his work, Slavery in the Roman Empire, went as far as to say: “The full opportunities for civilized life could only be fully used in freedom, no doubt, but slavery was an apprenticeship.”

Now as you can imagine, depending on the type of slave, his odds differed greatly depending on location, job, master's favor, etc. A slave in the tin mines in Britannia was likely doomed to an early grave whereas a slave who managed his Master's shop had much better odds.

But on to the details. What made manumission so attractive was freedman were given rights and privileges (legitimate children, transfers of property, marriage, wills, etc.), but along with these rights came responsibilies (obsequium - a freedman's obligations to preform operae [obligated services and inheritance rights] and maintain relations with his former master, the relationship was not unlike the well-established Roman patron-client system). These operae could take many different forms, perhaps, if his master fell on hard times, his former slave would be required to put him up. Generally, the conditions of the operae would "negotiated" during the manumission process. Although, you can imagine the slave had little leverage in the matter. Former slaves could not be charged in money by their former master, only services, however, if the freedman preferred to give cash he could. It is also important to note that slaves who had squirreled away enough cash to buy their freedom did not have these obligations.

Freedmen, of course, had some restrictions, most notably not being able to run for public office or access to the prestigious priesthoods. Military service was generally a no-go whether it be in the legions or urban cohorts (A general rule, I am sure the Romans broke it if they were desperate enough). Also, punishments in law were more severe for ex-slaves. Their was also a social stigma attached to a freed slave. He was likely a foreign prisoner of war, separated from country, tradition, his family, and even his given name and birthday. Freedmen were endless ridiculed and mocked for these things by the natural born Roman citizens (often jealous of the ex-slaves connections, wealth, or new found position).

However, many ex-slaves also had a host of factors in their favor. Trade and manufacturing were mostly run by slaves or former slaves (tradition forbade good, moral Romans of the senatorial class from taking part, so they sent their slaves to conduct business). Their talents and skills for fabricating goods, importing and exporting, and conducting business served many slaves well in their freedom. The Imperial administration and bureaucracy was, in large part, run by slaves and freedmen. As you can imagine, with these conditions present, some former slaves rose to immense prominence whether by administrative or financial means. Just so some basic searches on Claudius' cavalcade of freedman (Pallas, Callistus, and Narcissus come to mind although I'm sure there are more for other emperors) and their various schemes for power. However, it is important to remember that many of the accounts of these men are horribly biased and are from men of elite families like Tacitus. These social elites, who had so recently lost a tremendous amount of influence in the collapse of the Republic, were wildly jealous of these former slaves and their meteoric rise to power.

That went on a lot longer than I intended, but I hope it was helpful. I, unfortunately, can't really tie it to more modern times, but hopefully someone else comes along and can add to this.

Some recommended reading if you have further interest (my post is heavily based off these readings):

Slavery in the Roman World - Sandra Joshel

Slavery in the Roman Empire - RH Barrow (a bit antiquated, but a good read nontheless)

Greek and Roman Slavery - Thomas Wiedemann (fantastic collection of primary sources)