Many Red Light Therapy devices only deliver visible red and near-infrared wavelengths. How significant for healing is exposure to far-infrared wavelengths?

The far-infrared can be quite meaningful, more than as yet appreciated by science. This entire portion of the electromagnetic spectrum is not considered by people who think of sunlight as simply warming, as a kind of comfort, and then the well-known ability to promote vitamin D biosynthesis in the skin exposed to sunlight as being the primary benefit. That is just the start of things. It is a major source of energy and general well-being to have frequent sunlight exposure, and the far-infrared can be every bit as valuable as the near-infrared, depending on the particular physiology one might examine.