There is no such rule in this yoga. In fact most people retain their memory as well as sanity quite well. Want of balance is never a desirable thing and far from being a sign of progress is often an indicator of an inner disharmony leading to madness. This is often due to vanity, lack of humility and sincerity though sometimes it may happen that there is a sudden progress in one direction while neglecting other parts of nature which may in turn lead to chaotic activity showing up as madness. This is somewhat like the phenomenon of genius wherein the nature shoots up in one direction at the expense of other parts leading to a breakdown or odd behaviors. But this is an exception to the general rule. The other aspect unique to this yoga is the inability of the body to bear the pressure of transformation from the higher consciousness. As such even in traditional yoga an upsetting can take place if the consciousness expands quickly not allowing enough time for the body and mind to adapt as we see in the case of Swami Vivekananda and Sri Ramakrishna.
As to memory loss it is quite common in the elderly due to the loss of brain substance with age especially if there is a heredity predisposition. Otherwise in general yoga tends to sharpen the intellect and memory due to conservation of energy and its sublimation towards the higher centres of the brain.
Affectionately,
Alok Da
