"In 1900, the Paraguayan chemist Ovidio Rebaudi, after whom Bertoni named the plant, studied the major characteristics of stevia. He succeeded in isolating two types of substances (both glycosides): one extremely sweet and the other bitter, resembling a digestive appetizer. Of the two, it was the sweetening principle that attracted more attention at that time, as is still true today.
A combined process........ is traditionally used to extract the glycosides from stevia. However, the glycosidic extract has a pronounced bitter aftertaste that is responsible for many of the restrictions on the use of stevia as a sweetener."
Braz. J. Chem. Eng. vol.17 n.3 São Paulo Sept. 2000
I personally do not like the after taste of Stevia and try not to use it, but only for this reason.
Some toxicological work on the extract has had some adverse effects of the fertility of male rats................perhaps this is what the fuss about. David