I’ll comment to the link article (if there are better/other articles, I’ll read them too, but I already have to search other stuff for work)…
I’ll start by saying that very little is known about anesthesia in animals, especially compared with humans. This is even more so in exotics. There are no large scale studies, those that have more than a case report have still very few subjects, and sometimes there is nothing on the species you’re dealing with… So you adapt and infer from the closest species you have information about.
Seventeen and sixteen are old ages for ruminants in general, I don’t know exactly where the onyxes in the article fit… And although zoo animals get somewhat regular checkups, they are not observed as closely as some companion animals. They may have had some underlying disease that made them riskier candidates for anesthesia.
In the case of the sloth… Again, 28 years seems long-lived for a sloth. And a surgery for repairing twisted spleen with gastric dilatation and volvulus is a risky surgery that is done in dogs, and they carry a poor prognosis. In dogs, where there is more known about this condition and surgery, the odds are still crappy. Please put this in perspective when reading things like this.