I don't think treatment is a sound line of criticism of 'lumping', it's quite possible to have an overarching nosological definition while having different treatment regimes for sub categories of illness.Isn't one of major flaws in the fnd arguement that lumping many conditions under one roof with just one treatment option pyshc and physio.
Plus there is non exsistance of pathophysiological only a presumption that that the body is coverting thoughts and emotions into physical synptoms
Lack of pathophysiological evidence for FND doesn't help in excluding ME/CFS from an FND 'lump' because there currently is no certain ME/CFS pathophysiology. Clearly the FND adherents have some definitional issues, having allowed/accepted common ground with the wholly psychologically orientated Body Distress Disorder enthusiasts like Per Fink but I don't think that's enough to wholly undermine the nosological argument.