When is Mindfulness Based Cognitive Therapy not useful, or bad?
The mindfulness stress buffering account (Creswell, Lindsay, 2014) suggests that mindfulness practice  buffers acute stress reactivity, therefore it has more impact on health outcome of a high-stress group than a low-stress group.

Creswell, J. D., &amp; Lindsay, E. K. (2014). How Does Mindfulness Training Affect Health? A Mindfulness Stress Buffering Account. Current Directions in Psychological Science, 23(6), 401–407. https://doi.org/10.1177/0963721414547415