I just stumbled across the article Mindfulness therapy comes at a high price for some, say experts. Even when mindfulness is practiced correctly, side effects may occur, like feeling lost or disconnected from the world. What could be the cause of the side effects? Do these techniques deviate a lot from the Suttas or orthodox practice? Do the staff have the necessary training and awareness required to give meditation advice (Kammaṭṭhāna)? Can something be wrong with how the clinic administers the training / teaching?
This is a real phenomenon and it is discussed in the Visuddhimagga and the writings of the Mahasi Sayadaw. Some of the stages of insight knowledge can potentially manifest themselves in this kind of way. I think for most people these stages are relatively easy, but I suppose for some they could be much more difficult. Just from my own practice I can say that these particular knowledges can be very mild
It really is just the result of seeing impermanence on a direct level. When one has a lot of attachments this is not a fun experience because one is holding onto things while at the same time seeing the suffering involved in clinging. The answer to this is that one needs to accept that this is part of the practice and a part of giving up attachment, and continue to practice. Eventually one will let go and move on and progress.
This is actually one of the reasons why I personally have reservations about practicing mindfulness meditation in a purely secular context. The teacher probably won't know about this and without understanding general Buddhist doctrine it isn't possible to understand how to move on.
The article refers to side effects of the meditation practice on people with very deep mental illness. Mindfulness certainly is good for anyone, but meditation can be harming.
Normal people could experience boredom and sleep at worse. People with depression and anxiety have very sharp minds, and meditation (not mindfulness) could possibly worsen their state.
I think the problem they are trying to address is that on meditation you leave people to look and feel for themselves, and it is not safe to leave these mentally ill people "wander" on their thoughts and feelings by themselves.
My main concern about mindfulness, that some describe as meditation without the Buddhism, is that is construed to be a physical tool to make people happier. Without the practice of compassion for others and dedication of one life to some degree to helping others, mindfulness can degenerate to a feel good thing. But I do not condemn it. It is a start that can lead those with real inner yearnings to a meditation practice. It is baby steps, but there are so many babies in the world, it certainly has its place.