This is a good point. I would like to see some academic responses to Britton's research, but so far she has had many papers published in peer-reviewed journals detailing the various harms of mindfulness (experienced by a minority of practitioners, albeit).

I also wonder whether, even if you have a 'proper understanding and effort' involved in the practice, as you say, whether some at-risk individuals may still feel greater emotional distress. I guess at face value, it seems hard to imagine: if mindfulness is (partly) about being non-attached to negative emotions and thoughts, then how could mindfulness be harmful to anyone, and not only be helpful? One potential harm that comes to mind, however, is that meditating for a long time may lead to intense altered states, such as dissociation and depersonalisation, that some individuals may feel persisting after effects from.