Harmony in the pseudepigraphal writings?

It is not uncommon when talking to someone about the authenticity or composition of the Biblical canon to for the subject of the so-called "lost" gospels to be brought up. An explanation of the process used to determine canon, specifically the criteria the criterion of cohesion with the rest of the Bible can, among the more skeptical, bring about the claim that the early church was guilty of suppressing documents showing accounts that "show the truth" about the claims of Jesus or show Him in a negative light, etc. One would expect, if the claims at truth are indeed valid, these pseudepigraphal writings would show some degree of harmony within themselves, both the early (The Gospel of Judas, for instance) and the late (Gospel of Thomas). This is equally true for Old Testament non-canonical books such as The Book of Enoch. Harmony within these writings would support the truth claim and make a reasonable argument for their content. Disharmony amongst themselves would lessen the credence and support those which are harmonious as being true. Therefore, to what degree is there harmony among the books left out?