A very interesting book.
These were two systems of Unfreedom. American Slavery was more brutal for those enslaved. American slaves had work obligations about 3 times as high as Russian serfs. Russian Serfdom was more extensive. In USA 1790, about 18% of population was enslaved. In Russia, 50% of population was enserfed.
In America, African people were enslaved. In Russia and East Europe, serfs and nobles belonged to the same ethnos. In Russia and East Europe, nobles were small minorities. They lost their property during Socialist Revolutions.