"Serfdom is very different from slavery."

And Mac is very different from a PC, but they are still personal computers that pack some computing power...

My wider point was about unfree labor in pre-industrial conditions. There were many serf uprisings in Central Europe, which indicates that being a serf was sometimes very hard to bear.

We are so used to free labor nowadays that we can't really imagine a world where the vast majority of the population is physically subjugated to some lords.

My point is that Roman slavery was a lot more brutal than what was required by the lack of technology.

I beeeive (I cannot find hard data, although things like the Doomsday book should have recorded some snapshots of it) while there was a large serf population, it was not the "vast majority" as there were also lots of free peasants.

Yes, it was a hard life, but far better than their equivalents would have suffered under Roman rule.