Varangians and Janissaries, for example.
Also, how come those slaves didn’t revolt (considering their equipment and role in the military)?
I can't speak myself on this but this thread may interest you: https://www.reddit.com/r/AskHistorians/comments/4h39gp/what_were_the_differences_between_the_mamluks_and/ which has an answer from /u/Castabar.
Most importantly, the concept of Janissaries and Mamluks as "slaves" is a huge generalization.