Reddit reviews Methodical Realism

>only that they cannot establish with certainty

Knowledge "is the grasp of an object"; therefore we must begin with objects, not ideas. Thought "is only the consciousness of an act of knowledege". I'm going to take a long shot here and identify your disagreement as stemming from some idealist notion that begins with thought as the first principle of being. To cure that, I recommend this book.

>is irrational

Not quite irrational, but flawed in their reasoning. They have reached an incorrect conclusion as it is all too easy to do.