I don't see this as an argument against such legislation; consider that many physical crimes are also hard (or even impossible) to prove beyond a reasonable doubt, considered case-by-case. Rape very often qualifies here, as does the mens rea of various other crimes, which may rely upon testimony. Both actus reus and mens rea are required for a conviction, and while the actus reus may be easier to prove (but again, in many cases not beyond a reasonable doubt), we do not abolish the role of intention in the justice system simply because it's hard to prove.
Accusations of physical harms can also be concocted as bullying tactics too, in which the harm was suffered as a result of either a self-inflicted injury, or inflicted by somebody else. Such cases can be thrown out due to insufficient evidence. I see no reason why the same cannot be said for a subset of mental harms, in which there are equivalent doctors available to use their expertise to judge the harm.