As I understand it there are problems because of the specific use of the word in the US to refer to graduations of developmental disorder in the 19th/early-20th century. I don't know if this was used in the UK, but in the US medical context "imbecile" was someone who never became able to communicate verbally, "idiot" was someone who never learned to read because of general developmental disability, and "moron" was somebody who was literate but still considered to be developmentally disabled.