Distant attunement is possible but a direct attunement will work and perform better. Direct touch is important to transmit the metaphysical energies through the head and hands etc. Moreover, clients feel more convinced and confident when touched by the practitioner. They are likely to feel surer and convinced about the working of the metaphysical energy that passes from the practitioner to the client.
About distant attunement, it can be said that it works but not everybody can do it. In case of direct attunement, the newcomer into Reiki has a background of contemporary healing experience in which everything is done direct. So, it requires and necessitates a direct experience to initialise him or her, and once the client has experienced the power of Reiki, healing can be done remotely. In the beginning, the client has physiological needs as well.
It is important that a client goes to a practitioner who knows and is an expert in distant healing. Against it, in face-to-face healing and attunement, one can see what the practitioner is doing. He or she knows that the practitioner is actually doing the healing and not anything else. Whereas in distant healing, one can only rely and hope that actual healing is being done. There is no direct way to confirm that and there is the bridge of faith to convince oneself that he or she is being healed by the expert.
Distant healing is also less time taking and the expert can give it to many persons at a time. This means the focus of the expert on one person or his or her problems is comparatively less than what would have been otherwise. It follows that the degree or extent of the healing may get affected.
Another reason for direct attunement is that other aspects of Reiki such as teaching, practice and coaching are also very or equally important. Reiki is a complete healing way with different aspects and all the aspects must be learnt by the person for being able to impart training to others. There are queries to be answered during the practice session which comes up spontaneously which is not possible otherwise.
The way of direct interaction between the trainer and the student will take time which is not possible in indirect attunement. Only an in-person communication will ensure a real understanding of each other and the solution. Training in person helps because the trainer can see the student performing and suggest improvements or validate it.