Is it hard to get approved for disability with Chronic Fatigue ?

I took part in an online discussion recently and the individual who initiated the discussion professed a certain belief. Basically, he thought it should be easier to get approved for social security disability or SSI on the basis of a medical impairment for which the cause is not completely known, such as chronic fatigue or fibromyalgia. Is he correct in this belief? Is it easier (compared to