I find it hard to imagine how places like Slaver’s Bay and Volantis have their armies made up mostly of slave soldiers. Not only do these places employ slaves as bodyguards or cannon fodder but have entire armies comprised of slave soldiers. They also have slaves making up the police force of their cities (such as the Tiger Cloaks).

Now I’m not much of a historian but I find this notion ridiculous because in the ancient world slaves were rarely used in battle, only the most desperate masters would arm their slaves and give them permission to kill. This was mostly due to the fear that giving slaves weapons may embolden them to turn on their masters. Besides the Unsullied who are conditioned to be 100% loyal, there is really no reason why the governments of Volantis or the city states of Slaver’s Bay would trust their slaves enough to allow them to make up their military or police force given the immense risk that would impose.

Also the Volanteen fleet that’s sailing to Meereen which has its rowers made up of slaves is another example of the author converging from real history. In the ancient world oarsman weren’t normally slaves and more often than not were made up of freemen or citizens. This was because rowing (especially on warships) required a considerable amount of skill and not just a strong back. The image of prisoners or slaves chained to the oars of warships and forced to row while being whipped is a work of Hollywood fiction. 

I wouldn’t necessarily call them unrealistic, per se. Exaggerations, certainly, but there are historical examples of slave armies. The Mamluks were slave-soldiers who served in Egypt, Syria, Iraq, and India. Likewise, the Janissaries were slave-soldiers who served the Ottoman Sultans from Persia to the gates of Vienna.

The simple fact that it’s difficult to trust slave soldiers doesn’t mean people won’t necessarily use them. The Mamluks for example were ferociously effective in fighting off the Crusaders in Egypt and Syria – despite the fact that they seized control of the Egyptian Sultanate in the process. The Janissaries served the Ottomans well in battle, but they also overthrew Sultans whose policies they disliked or who didn’t pay them enough money.