Matters are different when we are dealing with degenerative conditions like Parkinson’s or the dementias, or when we are working against continuing insults to the system, as may be the case in the autism spectrum. In such cases the training needs to be continued at some level over time. Allergic susceptibilities and food intolerances make it more difficult to hold the gains. Poor digestive function will pose a problem, as does poor nutrition. A child living in a toxic environment (in either the physical or the psychological sense) will have more difficulty retaining good function.
If the problem being addressed is one of brain disregulation, then the answer is yes, and that covers a lot of ground. Neurofeedback involves learning by the brain and if that brings order out of disorder, the brain will continue to use its new capabilities, and thus reinforce them.