I've read recently that the Ottoman Empire got most of its eunuchs from one place: A Coptic Christian monastery called Abou Gerbe. The priests were supposedly renowned as experts in castration and their eunuchs fetched a high price. The method they used supposedly had in a ten percent recovery rate: in other words nine out of ten boys did not survive castration, if I read that right.
This seems like an astonishingly high morbidity rate. Is it true? Why did so many die? Was castration this risky everywhere or was it something about the methods these particular monks used?
Yes it is rather high, that would be the highest I've heard. Usually the given number is 1 in 3, but some (contemporary, 19th century) Western sources will give estimates as low as 2%, which I think is very low. I tend to lean to 1 in 3 as the more reasonable number, maybe an 80-70% survival rate. Curiously, one contemporary eunuch scholar named G. C. Stent, working with Chinese eunuchs in the 19th century, had similar troubles finding out the "real" number of fatalities. Most people quoted him quite low, but he was dubious (as am I) of anything too low.
The reason it was so dangerous in this particular instance is that black slave eunuchs were "complete" castrates, which means their penis and testicles were cut off. There was an extremely dangerous period when the urethra was healing when they were at a very high risk of death. Descriptions of castration date from Middle Eastern sources in the 300s to Western sources in the 19th century. One particularly reliable source is Clot-Bey, a Western surgeon who recorded these practices. (He gives a death rate of 1 in 4.)
Directly after cutting off the genitals, the castrator would insert a plug into the urethra and the child would not be allowed to urinate until it healed. In the Chinese tradition this was 3 days. At the end of three days the plug would be removed, and if they couldn't urinate, they would die. Provided they recovered they would still face numerous problems from not having a penis, including a high risk of UTIs and incontinence for some of them. Sometimes a plug was worn permanently to help assist with continence.
Methods for Chinese eunuchs were extremely similar (which I why I have mentioned them here), but eunuchs in traditions where there was no removal of the penis (Italian castrati most notably to us) had much lower death rates. Death rates for Italian castrati are hard to come by, but it was considered a minor surgery at the time.
The best source for Middle Eastern eunuchs is Eunuchs, Caliphs and Sultans by David Ayalon.
Hope this helps! (I'm going to mark your question NSFW because my reply is pretty plain worded.)