There are numerous objections to middle knowledge of course so it's not all sunshine and roses. But here I want to raise another potential objection. Perhaps that's too strong a term, actually, it's more like a potential problem. It's this: middle knowledge could explain virtually any scenario. But then you can't falsify it. This means you can't give any evidence that would rebut it. I say this is just a problem and not really an objection because you have to define "evidence" pretty narrowly to make it work -- as mentioned, there are plenty of objections to middle knowledge that have to be dealt with, and these objections could potentially refute it.
Anyway, my objection -- sorry, my problem -- can perhaps be illustrated by looking at some essays defending middle knowledge by William Lane Craig that specifically use it to explain Christian doctrines. The two essays I'm thinking of are:
"Men Moved By The Holy Spirit Spoke From God": A Middle Knowledge Perspective on Biblical Inspiration
So in these two cases, Craig is showing how middle knowledge uniquely explains the doctrines of a) the perseverance of the saints and b) the inspiration of the Bible (which could easily be a gateway to another essay giving a middle knowledge perspective on biblical inerrancy). Well and good. But then, it seems to me, you could write similar essays on other topics. For example:
"Upon This Rock I Will Build My Church": A Middle Knowledge Perspective on Papal Infallibility
which I presume Craig would not approve of as he is a Protestant (as am I). But such an essay could certainly be written. Of course Catholics could accept such a view, as long as they accept middle knowledge in the first place. But then what if I wrote an essay like this:
"The Governing Authorities that Exist Have Been Established by God": A Middle Knowledge Perspective on the Divine Right of Kings
Again, such an essay could be written, such a position could be defended by appealing to middle knowledge. My point is that it's difficult to see what restrictions we can put on this type of explanation. Presumably, someone could say the restriction would be biblical doctrines, but both of these positions have been defended by, I presume, honest and well-meaning Christians as biblical. Once you open the door, you're going to have people come in that you didn't invite.