I have deep misgivings about mcmindfulness taught by those with little wisdom and appreciation of the Dhamma. But small doses of the Dhamma administered skilfully can have profoundly beneficial effects, I feel.
Invite Shonin back here to participate in this thread, given that he is quite knowledgeable about therapeutic mindfulness practice. I would very much welcome his input.
Let me just say first that since the birth of our son I've been pretty engaged with that and with other things and I've not really been practicing with any sort of regularity, in a secular context or otherwise, but I'll throw in my contribution if you're interested. (Things are going swimmingly well btw). Also, I'm actually on holiday at the moment so I've only ready part of this thread and I'll keep things brief.
I would say that my experience matches Kelly McGonigal's pretty closely.
It is possible to just watch the breath as a stress-reduction technique. However, Mindfulness Based Cognitive Therapy and MBSR are not just that, they are Vipassana techniques with different terminology and a different theoretical framework. People do have profound experiences. For example, on the last stress reduction course I did at work (pretty low-commitment), one of my colleagues told me that her realisation that 'she was not her thoughts' was 'a life-changing experience'. Someone else on my own course told me he had encountered 'the ground of his being'. The people who come to secular mindfulness are a slightly different demographic from those who might practice Buddhism. And typically they don't have beliefs in rebirth, for example, and thus their motivations are not related to that. However, I've never had the sense that their motivations for doing so or that the techniques they are doing are more 'shallow'. They are practising Vipassana, insight meditation.
And people do need guidance, both in order to have such breakthroughs and processing them afterwards as well as a more general support network. That's what teachers and 'sanghas' (secular and religious alike) are for. I think it's hard to teach yourself. I think that having the benefit of Jon Kabat Zinn's (for example) insight on audio recordings and books helps, but having a good real life teacher is generally the best, at least at the start. (But that is not to say that it is harmful or pointless). My secular mindfulness teachers were very experienced therapists and mindfulness practitioners, both highly skilled at the reflecting back on what people offered and at drawing out insights. And at guiding people in their sitting to understand the difference between 'doing' and 'being' and so on. They were more effective at doing so than many Buddhist teachers I've practised with. It varies greatly from one individual to the next, but some people are very good at chasing their own tails and it's really challenging (even for an experienced teacher) to break that. It also helps to have a support network ('sangha') for ongoing commitment and practice.
It's the first time I've come across the term 'McMindfulness' but it's existence doesn't surprise me. However, I think it speaks far more about the contempt that some religious practitioners have for the secularity of such practise (of which they may have little or no experience) than it does about the value or depth of secular mindfulness. It's a little saddening sometimes, but knowing human nature, not that surprising perhaps.
Hope that was helpful.