I think the crux of the argument from those who have honestly evaluated this is that the NKJV often makes translational choices of undisputed passages in the same way what is more like some of the other more modern translation.
In a good number of cases the differences in translating in the NKJV is like what was in some pre-1611 English Bible such as the Geneva Bible. I could list many pages of places where the NKJV is exactly the same or very similar to the Geneva Bible while the KJV is more different.
While the KJV updated, made simpler, or revised a good deal of what would be archaic language in the Bishops' Bible, they kept some of it where the Geneva Bible or some other pre-1611 English Bible already used language that would be considered more up-to-date today.
KJV-only advocates tend merely to assume or speculate that the NKJV must be following some modern English translation based on the Critical Text. There ignore the possibility that in considering what some of the archaic language would mean or say in standard present-day English the NKJV translators could reach the same conclusions as the other translators. In addition, the NKJV also likely consulted some of the same Hebrew-English lexicons and the same Greek-English lexicons that may have been used by those other translators.
The KJV translators could not consult those lexicons since there were no Hebrew-English and Greek-English lexicons in print in their day. However, the KJV translators did consult Hebrew-Latin lexicons and Greek-Latin lexicons, and those lexicons sometimes had as the definition of the original language word the rendering of the Latin Vulgate. Some of the Hebrew-Latin or Greek-Latin lexicons available to the KJV translators were made by Roman Catholics such as Santes Pagninus. The guilt by association arguments that Gail Riplinger attempts to use against modern lexicons could also be used against the ones used by the KJV translators.
While I have not seen any convincing evidence that the NKJV translators borrowed any renderings from modern translations from the Critical Text, even if they supposedly borrowed a few, how would that actually be different from the KJV translators borrowing a number of renderings from the 1582 Roman Catholic Rheims New Testament based on the Latin Vulgate?
raying: