IIRC there's material in the books indicating that there have been serf revolts - Central Africa (the 'police zones' as they call it in the books) is basically under permanent martial law, and you get the impression there's some parts that they don't bother pacifying, they just make sure nobody ever comes out. And didn't Eric von Shrakenberg's brother die during a serf revolt? In fact, that's the main driver between the decision to engineer Homo Servus and conquer the planet, because they know that as long as there's free parts of Earth they're on a ticking clock before there's a huge revolt. I mean, it's author fiat, but he does try to explain it. Sort of.
Though mind you, the serfs we see are depicted in a horribly racist way - most of them being too scared or too in love with the Draka to revolt.
Also, Stirling seems to horribly misunderstand classical slavery - it seems like 'Well the Romans had loyal slaves, so why not these guys?' Missing the fact that Roman slavery was something that, though brutal, gave the hope of freedom - there was always the chance a slave could manage to buy their freedom, or do such a service to their owner that they'd be freed, then once they were freed their children or grandchildren could become citizens one day. In order to avoid a slave revolt, there needs to be the possibility of the slaves eventually ascending to the free and privileged class themselves, that way they'll have an incentive to buy into the status quo...and even then, there'll still be rebellions! Draka-style slavery doesn't leave any kind of avenue for that because Draka serfs can never be free.
Yep...