The answer is not easy to provide because when it comes to any theological position, it necessarily depends on ones perspective or personal inclination. For instance if one is charismatic in perspective one would say that their teaching on tongues was wrong. If one were Adventist in orientation, then one would admit that their teaching on the Afterlife was correct.
Probably the best view to take in research such as this is to test them according to their own beliefs.
1 They claim to believe in the inerrant nature of the Bible, yet they claim, at the same time that it is not inerrant because it had been tampered with. Hence their need to "restore" God's "name" into the NT. Either the Bible is inerrant and therefore untampered with or else it is in error, thus needing to be restored. In appearing to stand for an inerrant text of scripture, they actually are advocating a conveniently eclectic text. Thus making it capable of saying, like a pliant whore, whatever it is they want it to say
2 Most of what they believe on various doctrines is duplicated by other groups, but those which are unique to them have, at one time or other been either discarded, or revised in such a manner as to suggest that no revision has actually taken place. The 1984 teaching of Christ's "invisible" presence is a prime example. When it was dropped, and 1914 advocated, it was done under the subterfuge of "progressive revelation" That it was in fact a contradictory revelation was quietly and successfully ignored.
3 Not only have their own doctrines needed revision, but their words as well. Their own literature, established as authoritative when written with the imprimatur of the WTS, has often been found to be wanting. For instance, when the "Aid" book was written, back in 1971, it said on page 584, under the subject of "firstborn" that the word could mean "pre eminent". ""David, who was the youngest son of Jesse, was called the "firstborn" due to his elevation by jehoober to the Preeminent position" However, much to the chagrin of the WTS leadership, this would mean that Jesus was not the first created being by jehoober, but that He was Preeminent over all creation. Col 1:15. So those words concerning David had to be rewritten. The "Insight" book, published in 1988, in volume 1, pg 836 now says: "It seems that jeboober was referring prophetically to the one foreshadowed by David" Gone is the offending phrase that "firstborn" can mean "Preeminent" So, having been written in 1971, it took the WTS leadership seventeen years to realize that indeed, the "Aid" book was in fact not an aid to Bible understanding.
The average R&F member of the WT movement is encouraged to believe that these later publications are indeed examples of "advancing light" when in fact they are devious, and ultimately dishonest attempts at covering up theological embarrassments. It is only when, like all of us, a R&F member is moved by an impulse to research that the full extent of this deception is uncovered.
So, one can say, with a confidence borne from exacting investigation, that none of what the WTS has ever said has been found to be true. Like the master they serve, under the guise of using this hybrid word jehovah, there is no truth in them. [Jo 8:44]
Cheers