Leading scholars say the Greek manuscripts of the other versions, mostly the Sinaiticus/Vaticanus, also known as the Westcott &Hort, are not older and more reliable than the Greek manuscripts the King James uses, the Textus Receptus and also known as the Erasmus, and they say that the Sinaiticus/Vaticanus manuscipts are corrupted. I quote James I. Packer, Merrill C. Tenney & William White from page 76 of The Bible Almanac, Nelson Publishers: "the early textual history of the Greek New Testament was more complicated than [Westcott & Hort] supposed. These two Anglican scholars assumed that the earliest text would be the purest, with the least difficulties and the simplest readings; but this does not now appear to be the case. Also, Westcott & Hort know nothing at all about papyri or their readings. Therefore, they were unsuccessful in tracing the text back beyond the second century by the aid of text types"
Dean John Burgon, a highly respected Bible scholar of the mid to late 1800’s, wrote of these manuscripts, “The impurity of the Texts exhibited by Codices B and Aleph [Vaticanus and Sinaiticus] is not a matter of opinion but a matter of fact.”. He also said that the Codex Sinaiticus, as well as codices Vaticanus and Codex Bezae, were the most corrupt documents extant. Each of these three codices "clearly exhibits a fabricated text – is the result of arbitrary and reckless recension." The two most weighty of these three codices, א the Sinaiticus, and B the Vaticanus, he likens to the "two false witnesses" of Matthew 26:60.
There are many leading scholars that agree with this.
Another scholar that has exhaustive studies on the TR and Sin/Vat is Rev. D.A. Waite, Th.D., Ph.D. He contends that the Vat/Sin is corrupted. He has one of the most vast resources and studies it seems to me.
The contention with a lot of these scholars is that the corruptions were done by the Gnostics, of those who say Christ did not come in the flesh, see 2nd John vs. 7. The differences with the TR are all the same subject, they are in passages that talk about the flesh and sin effecting salvation. The Sin/Vat says the flesh and sin do not affect your salvation, and the TR (which the KJV uses) says it does!! That's because the Gnostics changed those passages creating the Sin/Vat because they believed that Jesus did not come in the flesh, that Jesus was just a Spirit, so He saved your soul not your flesh since we all die, so if you sin it does not effect your salvation.