I've practiced mindfulness and meditation for more than a decade.

Mindfulness, as I've learned it, means being aware of your thoughts and feelings, introducing positive thoughts and feelings, and replacing negative thoughts and feelings.

Meditation is also meant to be positive. You start with short sessions. The idea is to slow down and eventually stop your thoughts.

From what I've heard, a lot people just fall asleep at Vipassana retreats, due to the long meditation sessions, which would not be very useful. The way I learned meditation is that short, focused sessions are better than long sessions where you don't focus / just fall asleep. Based on this article, it sounds like these extended sessions might even be dangerous for some (almost like sensory deprivation or psychedelics can be in some cases).

Meditation on its own is not dangerous. The idea is to feel more grounded and centered after. The idea is to focus and develop your concentration. Otherwise, it's just "spacing out", which I've always been told is not meditation. If you space out and start to go into weird states of mind, that is not meditation, that is just dissociating / falling asleep / etc. There is a difference.

There's nothing wrong with slowing down your thoughts. If people think that slowing down your thoughts is dangerous, to me that just sounds like the ego not wanting to lose its grasp. Egos can be almost as crazy as someone having a psychotic break. It's fine to let yourself rest in your natural state for a little while. Nothing wrong with that. In my experience, that actually makes you more balanced then someone overthinking their whole life / not being aware of what's going on inside themselves.

I'm not a doctor, so get help if you need it, but one suggestion for a meditation practice is to start with 15 minutes a day, morning and evening. Set aside some space in your home and sit quietly for that period of time. Build up from there. Sitting in nature can also be helpful. If you start to feel weird, try to use your mindfulness practice during the day of focusing on positive things to get yourself out of it. Don't stay in a dissociated state, that is unproductive and potentially dangerous. If that doesn't work, stop and come back at a later time.

Meditation and mindfulness are meant to support one other. They are not exactly the same, but they are related. When you start to meditate you will become aware of how many thoughts you are having (most people go day to day not paying that much attention). With mindfulness, you practice directing your mind in a more positive way (i.e. neuroplasticity, you are restructuring your mind with a positive intent). This helps your meditations be better and more productive, and helps your life be better and more productive.

The way that I've been taught meditation and mindfulness are that they should be positive practices that support your life. If they are making life worse, something is off with that teacher or meditation school. Your own results can inform you if a teacher or meditation school is positive or not. If you're not getting positive results, that's not a good sign. These practices should make your life better.