I suspect that one difference between the classic treatments (like the
Sector General books or "Expedition to Zhodane") and the more recent
nightmare scenarios is that the technology used was bulky, finicky, and
really only worked on one (presumed willing) subject at a time (and took
some time to do so).  At worst it's a psychological thriller and/or
"Manchurian Candidate" scenario, not the sort of mass mind-control where
you rapidly assimilate, er, re-write an entire country or world, then
use them to take over the next, then the next, and so on.

And with specific regard to the SG books, per the OP's question, that's
a setting where sentient life is so ubiquitous and varied that they need
an equally complex classification system, and no doctor can possibly
know them all.  In Traveller, a doctor has to know how to treat a dozen
species, *tops* - more commonly, only two or three.  That's well within
the bounds of personal knowledge and reference materials.

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Kelly St. Clair
xxxxxx@efn.org