I agree about the variety of conditions dumped into the FND bucket. But we have seen enough published case studies of people who have been given an FND diagnosis and then have gone on to have worsening symptoms and eventually be diagnosed with a progressive neurological diseases to question the capacity of the medical system to always accurately identify progressive conditions. For example, there was the woman with Creutzfeldt-Jakob Disease who was initially diagnosed with FND (with the case study authors continuing to insist that she had an overlay of FND, even after the identification of the CJD, even as her brain disintegrated. I still find that case study one of the most astonishing medical accounts I have come across.)
A family member had a malformation of the blood vessels on the surface of the brain - as I understand it, an artery not connecting up properly to the capillaries, but instead bypassing those to shunt the high pressure blood into a vein. Such a malformation has all sorts of causes - congenital, physical trauma and disease. The vascular malformation caused some death of brain cells that had been served by the bypassed capillaries and, so, a loss of function, some of which was regained as neural plasticity allowed other parts of the brain to take over that function.
The structural problem also caused swelling where the vein became leaky under the high pressure of the shunted blood supply. The swelling, pressing on the brain and its blood supply, would periodically get worse, typically after some activity increasing blood pressure, and then reduce. That produced a pattern of fluctuating symptoms, including a loss of function in a hand, cognitive issues and delirium.
In my family member's case, the malformation was not diagnosed straight away, and in fact may not have ever been. He was very fortunate that someone took another look and arranged the right sort of scan. To my knowledge, he was never given an FND diagnosis. He is of the age where strokes are expected, and that is the diagnosis he was given. If he was a young girl with the same problem, maybe the outcome would have been different.
This experience underlined to me the ridiculousness of suggesting that a fluctuating pattern of symptoms is evidence of conversion disorder. Stress, whether it be physical or emotional stress, does have impacts on the body, and those impacts, such as an increase in blood pressure, absolutely can exacerbate neurological disease.
(sorry for a few edits for clarity)