If candida can "mutate around" nystatin, then it just as possible that they could "mutate around" natural products like oregano oil and such.
Just because something is only available by prescription does not make it inherently more "evil" than items that are non-prescription. I think that both could have the chance of not killing all of the various breed of candida a person may have, leaving the person with a case of only the breed that was resistant to the oregano oil, or nystatin, or whatever they took.
That's why I think it is important to do a multi-product attack.