I am very skeptical about anything being guaranteed, especially miracles… yet, many years ago I made the nine Monday novenas to Mary under the Miraculous medal… not sure what exactly they were called…and I DID get my prayers answered.
I wanted to meet the love of my life… and I did a few months later. I would not hesitate to recommend it BUT I would never say with 100% certainty that it will always work or go as you wish it to go.
I think that borders on superstition, and seems contrary to what our faith should be all about.
However, Jesus does tell us that we should ask God for favors and BELIEVE as if the prayer were already granted, and God will grant what we request… I think that takes a special kind of faith, that very few people have.
Many of the saints had enough faith to do this and God performed the miracles that they requested. I don’t think most people would ever go out on a limb like that and basically predict and believe enough that God would grant them such miracles. Then again, maybe these novenas are just that, a way for us to build up enough faith that God gets around to granting the miracle that we seek.