Priesthood blessings do not appear to heal people beyond what may be experienced through the placebo effect, and predictions and counsel given during blessings do not appear to outstrip the conscious or subconcious capabilities of whomever is acting as voice. I derive my confidence from the following data and observations:
I’ve listened to many stories online, from my own siblings, and other friends of blessings which were pronounced in good faith and under the influence of what they felt was the Spirit which did not come to pass.
There are no modern records of an amputee ever having a limb restored (“why won’t God heal amputees”). This suggests that truly miraculous healings do not ever occur. A poignant example of an injury that completely maimed an organ was when James Talmage’s brother was accidentally blinded by James with a pitchfork when they were younger. The brother confessed the absolute faith to be healed, but after numerous blessings by Apostles and Prophets that his sight would be restored, his eyesight was never restored and Albert died a blind man (see pg 30, right column). The inability of Apostolic Priesthood blessings to heal a completely maimed organ seems consistent with the notion that blessings only exercise influence within the bounds of the placebo effect and the probability distribution of what may occur via natural means, even if rare (for example, the spontaneous remission of some kinds of cancers).
Presumably under the influence of the same holy spirit that animates patriarchs and other blessing givers today, Patriarchs have made many promises to recipients that they would live to see the second coming and would “not taste of death.” These promises were not fulfilled.