There always a lot wrong with the NWT right from the start, but generally, considering they had no Scholars expert in the field of producing a new version of the Bible, they did well, often many renderings being better than rival versions. It was not therefore though a true "Translation", and still is not. It is a work based on the work of other Translators.
A couple of things that were wrong were its initial slavery to the concept of rendering the Bible "literally", often idioms etc from the originals were given as near to the original words as possible, and thus were ponderous and hard to read in English, but also the method lost the sense of the original idiom.
The second, and more important, and in fact dishonest error, was to change the meaning of the words in the original language, to suit their Theology. Of course many other versions are guilty of this too !
It is a pity that the latest Version does little to improve it as contender in the market for "Recommended Good Version" .