Doing the Jesus Prayer with the various breathing and other physical techniques can indeed lead to evil influences, thus it is normally required that one have an experienced spiritual father (i.e. director) to guide one through the use of these techniques. The evil influences are such things as pride and delusion, as well as some fairly nasty psychological effects. I’ve heard stories of people who either didn’t have a spiritual father, or did not practice the Jesus Prayer as ordered by the spiritual father (performing more prayers than prescribed) who literally went insane. This is because the spirituality of the Jesus Prayer, or more properly hesychasm, takes one through one’s entire life and reveals the truth about oneself. To paraphrase an overused quote, not all people can handle the truth. When one begins to see one’s sins and sinful way of life for what they really are, then we are often led to despair and driven insane by guilt and over-scrupulosity. Without the guide of an experience spiritual father how can we hope to avoid such pitfalls?
That being said, there is not universal agreement among the Eastern Fathers that the physical methods and breathing methods of hesychasm are useful. St Theophan the Recluse seems to have almost despised such methods, as did Ignatius Brianchaninov. For such as these the important thing is prayer “with the mind in the heart,” that is, focusing all of one’s attention and feeling, indeed one’s entire being, on the prayer and on the Person to whom the prayer is addressed. They also say that if one cannot find an experienced spiritual father, then one should study the Church Fathers attentively, especially the writings contained in the Philokalia.