The way your question is phrased implies that you think that sufferers of complex and poorly understood diseases should be assumed guilty (of fraud or insanity)

The way your question is phrased implies that you think that sufferers of complex and poorly understood diseases should be assumed guilty (of fraud or insanity) until proven innocent. If that is what you're trying to say, then I would reply that both the history of medicine, as well as the sheer amount of known (to say nothing of unknown) blind spots within current medical practice point toward a more credulous approach toward these sufferers, if you want to believe the truly sick significantly more often than you want to catch the fakers.