Not saying that peasants in Poland-Lithuania were in an enviable position, but this line of comparison is advanced by some popular non-historians: I'd say, to get some "cool" surface analogies to the US. Slavery would be more if there were an option of taking or leaving your peasants with property as you like. It's hard to imagine for the nobility to ignore the manpower in valuation. And there was still some actual slavery in Lithuania (small scale) and Russia, and also if you'd be kidnapped by Tatars and sold somewhere in the Ottoman Empire. The latter was a fairly common thing in some parts of the country.

Peasants, as opposed to slaves, were integrated into the civil society, although on very disadvantageous terms. Even on private land, they had their own judicial "councils" (look up 'sąd dominialny'), of course entirely on the mercy of the lord in practice. (There was also the state/crown land with marginally better conditions.)

All this didn't stray qualitatively from the broad Eastern European baseline. A Russian, Prussian or Hungarian noble wouldn't get punished for killing his peasant either, unless he'd seriously screw up his relations with the local regime. A French one pre-1789 would have to play his cards right. PLC got much flak in the period for peasant oppression because 1. it often didn't have raw political power to distract from it 2. there was the perceived hypocrisy in its political system, very loud about liberty, equality etc.

Great comment, thanks!