Reiki is capable of healing anything because it works at very fundamental levels of reality. Even though the capability is there, this is not what always happens. The limits to Reiki seem to be in the recipients willingness to cast off old habits and patters, to accept change and to accept healing. The level of reality where Reiki operates is the underlying energy structure of matter, as the physical matter we see around us is a solidified form of energy (Remember Einsteins famous equation which says that each unit of mass is equivalent to a certain amount of energy). At the level where Reiki functions, anything can be changed because all is fluidlike and is very malleable. Or so is the theory and so is the experience of Reiki practitioners. Emotional difficulties are just as healable as physical ones since emotional issues are even more directly present in the energy structures.
In this context, the word Healing has a somewhat different meaning from the widely accepted meaning. The widely accepted meaning for healing seems to be curing of symptoms, for that seems to be what medical doctors and the like look to do in their practice. The other meaning for healing, used in the practice of Reiki as well as other related areas, is the return to greater wholeness. There is an ideal form each of us has, this ideal form being the highest and clearest expression of who we are. Pain or disease comes from any deviation between the persons current form in the 3D physical world and this ideal form. Healing, then, is to bring this physical form into closer alignment with the ideal form.
The way we aquire deviations from our ideal form is to accept limitations into our life. Most of this comes from early childhood because that is the phase of life where we are the most open and inquisitive about life. A limitation may be a parent yelling "BE QUIET!!" enough times that the child learns to not speak. Another limitation may be a limp that continues longer after the physical injury has healed, maybe with phantom pains. These limitations are behavior patterns, eating patterns, physical limitations, imagined physical limitations, psychological, mental, or emotional ways of being, living, expressing or loving that is not in alignment with our personal highest expression.
In any healing the goal is to find the limitation, recognize the pattern, recognize where it came from, and let it go. Reiki accomplishes this by providing the recipient enough energy to step above (metaphorically) to see all that and have the courage to let go. This usually does not happen consciously as a result of Reiki, but sometimes it does happen that Reiki gives the recipient the conscious awareness of the pattern and recognizing where it came from. Our lives are a constant flow of patterns of activity (such as the pattern used to accomplish eating breakfast) and it is our choice to have these patterns remain stuck in limited expression, or to release the old patterns and try on new and shinier ones.
Reiki is also a gateway shining pure love into the universe. It is this love which allows us to transcend our wounds and help us remember our true nature.