Are Sinaiticus and Vaticanus reliable Greek manuscripts for English translations? The question is important because most of today's English Bibles use these text types as their translation basis, via ecclecticism.
John Burgon, Dean of Chichester, gave a harsh critique regarding the "Revised Version" of the English Bible (1881), and such criticism is valid today for modern translations using these manuscripts. His varied works include The Traditional Text of the Gospels, Revision Revised, Defense of the Authorized Version, The Last 12 Verses of the Gospel of Mark, among others. He was an expert in his field in his day, Textual Criticism.
Burgon has this to say about Sinaiticus and Vaticanus: "....Vaticanus and Aleph have within the last 20 years established a tyrannical ascendance over the imagination of the critics, which can only be fitly spoken of as a blind superstition. It matters nothing that they are discovered on careful scrutiny to differ essentially, not only from ninety-nine out of a hundred of the whole body of extant mss. besides, but even from one another. In the gospels alone B (Vaticanus) is found to omit at least 2877 words: to add 536, to substitute, 935; to transpose, 2098: to modify 1132 (in all 7578): - the corresponding figures for Aleph being 3455 omitted, 839 added, 1114 substituted, 2299 transposed, 1265 modified (in all 8972). And be it remembered that the omissions, additions, substitutions, transpositions, and modifications, are by no means the same in both. It is, in fact, easier to find two consecutive verses in which these two mss. differ the one from the other, than two consecutive verses in which they entirely agree."
He goes on to say, "In the Gospels alone Vaticanus has 589 readings quite peculiar to itself, affecting 858 words while Aleph has 1460 such readings, affecting 2640 words."
Elsewhere: "Lastly, We suspect that these two manuscripts are indebted for their preservation, solely to their ascertained evil character, which has occasioned that the one eventually found its way, four centuries ago, to a forgotten shelf in the Vatican library; while the other, after exercising the ingenuity of several generations of critical Correctors, eventually got deposited in the waste-paper basket of the Convent at the foot of Mount Sinai. Had Vaticanus and Aleph Sinaiticus been copies of average purity, they must long since have shared the inevitable fate of books which are freely used and highly prized; namely, they would have fallen in decadence and disappeared from sight." In summary- these two unicals are old and survived because, first, they were written on expensive and durable animal skins, and they were so full of errors, changes, and deletions, that they were never used by believers and seldom even by their own keepers in the monestaries. Thus they had little chance of wearing out.
In addition, if two witnesses disagreed with one another to the degree that these two do, they would be discounted in a court of law.
The works of Dean John Burgon are recommended along these lines, but we do warn against the modern "Dean Burgon Society" as it has KJV onlyism at its root, and anyone familiar with Burgon's writings would not have been KJV-only.