My answer may shock you, but yes, I see that in some ways, practicing some forms of Buddhist meditation without some understanding of what you are going to experience and if you have a pre-existing mental condition with no teacher to guide you can create feelings of great emotional stress and extreme anxiety. Saying that, I want to quickly say, this is only a tiny portion of those that attempt it and many people with mental disorders have found great relief from practicing Buddhist meditation. Obviously, much of what the first paragraph says is fear mongering, religious bigotry and just folks who have a enormous misunderstanding of our Buddhist practice.
I have experienced great fear while sitting in ZaZen a few times, and to be honest I was scared shitless. After you practice for awhile, some may start to see the realization of the emptiness of self and see some smatterings of unbridled reality. If one is unprepared for such an experience, feelings of great anxiety and fear can suddenly swell up, and this is perfectly normal and common. I think for the vast majority of practitioners this does not pose a great problem. However, the impact of realizing the emptiness of self, ego and identity should not be underestimated. When one begins to realize all that he or she took as self, identity and reality is stripped naked of the minds conceptions, not only can it become a life changing, beautiful event but it can also shake one to the their crumbling foundation.
In Zen, it is said that one must pass away before the attainment of enlightenment. They don't mean to actually die, but the idea or notion of self or ego dies. I think this is just as significant as death itself though. I realize this can be confusing. Perhaps this may help a bit:
If you think why we have difficulty in our life you will understand how to accept the difficulty. Before you are, you do not realize that we are one piece of water or one piece of universe. You have fear. You have difficulty because you have feeling. You attach to the feeling you have just now without knowing just how this kind of feeling is created.
So to dip the water from the river is to feel the water, the value of each individual, the value of the person who uses the water. At the same time when we become aware of individual feelings we have the feeling of the value of the water. So we…because of this feeling we cannot use the water just materially. It is living being. Originally it was with the big river. So if the water becomes one with the big river, the water will not have any feeling to it. It will resume to its own nature. By resuming to its own nature they have composure, they will be very glad to come back to original water. If so, when you die what feeling will you have? I think it is like water. We are like water in the dipper.
So if someone takes us to the original river the dipper…the water in the dipper will be very glad. So if we come back to original home we will be very glad. We will have composure there. Perfect composure. It may be too perfect for us just now. Now we have some….because we are so attached to the individual existence like this so we have attachment. For us, just now, in this way we have some feeling of fear or fear of death but after we resume to our true nature there is nirvana. That is why we say to attain nirvana when we die. It is same word. We say ‘to take nirvana’ is to pass away…to pass away is not so adequate expression. ‘To pass on’.
~Shunryu Suzuki, Zen Mind, Beginner Mind
We are dualistic, so what about the good stuff? When we seek truth with an honest motive, I think, although there may be some genuine difficulty, the realizations and understandings we have can transform our life or more importantly our mind's view of life to be more in sync with reality. We see people everyday, from every walk of life show great benefit from practicing earnestly, with a keen mind and right understanding. Today even many psychotherapists have adopted some Buddhist techniques of mindfulness and concentration in tandem with a cognitive therapy treating all sorts of different types of patients. My Zen Master once said that the appearance of fear is an encouraging sign of a students progression in practice. Weather it be Vipassanā, or ZaZen or some form of Tibetan Tantric practice, I think these forms of right concentration must be investigated or perfected in conjunction with right understanding, intention, mindfulness and effort. Perhaps, think about it like building a bridge across a river with no planks to walk on.
The ironic thing about this is the eventual appearance of fearlessness and composure. Once one begins to see beyond the shadow of self, then fear will begin to fade from the mind like a wave gently rolling back into the ocean. While I think it is more than possible for one to progress their practice by themselves, perhaps even to the attainment of enlightenment itself, most of us are in need of a teacher to see improvement. They can guide us, be our lifeline, show us where we fall and encourage us when we despair. I can not stress enough how much great wisdom and substance a good teacher can bring to us, to ease some of our worries and to explain some of our own fears. If you have the ability, I encourage you to find a teacher/monk/master, one that suits you and your choice of practice. Learn from others online or around you about the men and woman out there that do offer instruction, guidance and discipline, and make an informed decision.
So, is Buddhism dangerous? Perhaps, to a few, under certain conditions with no guidance or understanding. More importantly however, the thrust and tradition of Buddhism can mean the ultimate extermination of fear. Everything we do in life can be dangerous, getting out of bed, crossing the street or taking a shower can be dangerous. I believe our greatest danger we have, however, is not to reflect upon ourselves. Fearlessness is that perfect diamond embedded inside the boulder of fear.
"The only reason we don't open our hearts and minds to other people is that they trigger confusion in us that we don't feel brave enough or sane enough to deal with. To the degree that we look clearly and compassionately at ourselves, we feel confident and fearless about looking into someone else's eyes."