Well, it's a given that any historical text is going to be subject to error and bias, or that any individual's recollection of events from decades earlier is going to be imperfect. (I think I saw Leonard Nimoy make a similar mistake of the order of events in an interview about his Mission: Impossible years -- he thought he worked with Lesley Ann Warren in the first of his two seasons, when in fact it was the second.) That's why one should never take any single source as authoritative in itself, but try to get a consensus, or at least a parallax, from multiple sources.
In the case of using the soprano early or Alexander Courage late in the series, it was ridiculously easy to confirm. If a fan can pop in two episodes and call shenanigans, it does tend to put the overall text into doubt.
However, I still find it to be the best making of Trek book out there.