The authenticity of these supposed miracles is vouched for by the same people that tell us there was only one Sister Lucy, and that the Third Secret was revealed about 20 years ago. They also tell us with infallible certainty that Paul VI is currently enjoying the beatific vision. Okay. 
So, the problem I have with the "authenticity" of the miracles is equivocal understanding of what is meant by "authentic".
Simply because you can rule out natural causes, including human fraud, and demonstrate that the phenomena were in fact preternatural, this does NOT rule out the diabolical. So, for instance, the docuмented phenomena at Garabandal do seem to be very much preternatural. But this does not mean it is of God.
Bishop Williamson states that the miracles cannot be disputed. Well, the Polish ones could be suspect. Those were both cases of a single individual going back later and allegedly "finding" the blood on "consecrated Hosts" that had been put away to naturally dissolve after having been dropped. What's very strange is that the two Polish ones are nearly identical ... both involving the exact same scenario, where a Host was dropped and then put aside to dissolve in this manner, only for someone to later find it with blood on it. That seems strange to me. Bishop Williamson urged someone to check the internet ... as if there have never been fake / fraudulent posts made on the internet.
But let's say that there's miracle that took place in front of a number of people who are credible witesses, and the blood/flesh samples were proven at a lab to be authentic blood and human heart tissue, what's to stop the devil from swooping in to introduce this tissue from some other source? Probably only a valid Blessed Sacrament (as I don't believe God would allow demons to tamper with it). Of course, if it's just bread, as many of us believe, it might be fair game.