To understand the answer to this question, one should understand that Reiki is a complimentary healing system based on the use of natural life force energy. Though it has tremendous potential to cure the diseases, yet, it follows the laws of nature like all energies do and treats the person accordingly.
Reiki is no doubt very effective for treatment of many diseases but it offers no guarantees and warrantees; no healing system does. The cure always comes from God, not a doctor or Reiki healer exactly like it happens in allopathic, homeopathic or any other medical system. Cure is always a grace.
Fundamentally, no healing system can guarantee any cure. While there can be miraculous cures through Reiki, at times there may be very negligible or no cure at all. Even otherwise every ailment is not curable. For example, a person who got polio in childhood and is now grown up adult with permanently distorted limbs cannot be cured. It is a condition that cannot be expected to get cured no matter what healing system is used. Similarly many fatal diseases at advanced stage may or may not be possible to cure. If during the treatment, the person passes away, we cannot say that Reiki failed because obviously, Reiki or any other healing system cannot prolong anyone’s life. We all know that sooner or later a time comes in everyone’s life when no healing system works and finally the life ends. The law of nature is that when a person becomes terminal, all healing systems and medicines become ineffective. Reiki also abides by this law and never violates it. It is important to be realistic in approach and not over expect from any healing system be it Reiki or whatever.
Basically Reiki is meant for personal development. As a healing system it should be taken as complimentary, not the alternative system. Whichever system sounds more useful in a given situation, should be used. For example, in case of an accident surgery might be the requirement and only option. Reiki can be used after surgery that will not only help in speedy healing of wounds but will also positively affect overall health. So, it is important to understand how Reiki works and attach realistic hopes only.