Generalization beyond n-back tasks had not been established, at least at the last time I looked into the field, maybe 8 years ago. There was always the hope that tasks that focused on specific core cognitive processes would lead to broad cognitive improvements, but I am not too confident about it.

One problem in this field has been weak control conditions..e.g. no train conditions. I often thought that a cardio control conditions would be useful. I'd almost guarantee that for most people, 60 minutes a week jogging would lead to better cognitive improvements than 60 minutes of n-back.