Maybe I am misinterpreting your second point, but there are absolutely unique and consistent dysfunctions of ME/CFS sufferers. One example is severe exertion intolerance - this is not present in most other diseases. Generally, if we exercise more than a small amount we are literally bedridden (sometimes hospitalised) for days or weeks. It's often used as one of the diagnostic criteria. Psychological illnesses generally respond very well to exercise, and ME/CFS doesn't. It's a good way to rule out ME/CFS. There are also consistent immunological abnormalities (low NK cells for example), and recently the metabolomics study.