Was is the testicles or the prostate that was cut off? Or both? Were there variations by region?
More can always be said, but there are some answers from /u/caffarelli
https://www.reddit.com/r/AskHistorians/comments/21btwr/how_were_eunuchs_castrated/
https://www.reddit.com/r/AskHistorians/comments/3d0rf9/what_was_the_survival_rate_for_eunuchs_in/