Not all predictions are equal. So I don't think that the fact that Spock makes a lot of offscreen, untelevised predictions means much; the predictions we see onscreen are not only the most dramatic, they are also the cases where predictions are the most important. As such, we should weight them much higher in analyzing Spock's competence than his offscreen predictions.
I could predict 365 sunrises and one asteroid impact for the next year, and if I was wrong about the asteroid impact, it would be good reason to call me a bad predictor, even if the other 99.7% of predictions were all correct.
And using a novel as a source for canon is dubious. You can do all the Sherlockian "pretend the novel really happened" you want, but onscreen episodes weren't written taking that novel into account.
This is actually supported by cannon. In Star Trek Voyager we get the impression from a few episodes where Tuvok talks in some detail about Vulcan mental practices and makes clear, for example, that they have a large number of highly specific categories of emotions that we don't have words for in English or, presumable, any Earth language. Things that we would lump under a single emotion have fine distinctions in Vulcan.
There is some precedent for this here on Earth. Within some Buddhist traditions, abhidharma texts are important and give detailed categorizations of mental phenomena, including emotions. These are meant, at least in part, to help practitioners better understand the nature of their suffering so that they may find liberation from it.
The fact that the Enterprise has survived for a long time may be due to the fact that captain Kirk overrules Spock in the areas where he is not competent (for example, when he estimates the probability of escaping from a black hole), while he is good enough in other aspects of his job.
The fact that Captain Kirk decides to ovverrule Spock's 99,999999 % predictions is strong evidence that he does not trust them.
[Epistemic status: tongue-in-cheek]
Spock has traditionally had a poor reputation among the aspiring rationalists of the early 21st century; for his poorly calibrated (even anti-calibrated) probability estimates, labelling of actions and emotions "logical" or "illogical", and difficulty with emotional and interpersonal relationships. I argue that such condemnation, based on televised and non-representative events, is ill-founded.
First, all credible sources agreee that the Enterprise did in fact survive innumerable and hitherto-unknown hazards, which makes it hard to believe the the First Officer (and Science officer) could be so incompetent. Observing the strong effect of selecting events for dramatised television, our confusion is explained away by Berkson's so-called Paradox: Spock's probability estimates appear to be anti-correlated with subsequent events, but this is only because the many cases where he correctly identifies an event as likely or unlikely make for dull entertainment and are therefore ommited from the record.
Second, while the destruction of Vulcan makes reliable sources difficult to find, Spock's World documents the philosophical/religious idea of cthia, translated as "reality-truth — seeing things the way they really are, instead of the way we would like to see them". It is unfortunate that, being a scientist and officer but not a linguist, Spock resorts to the concise "(il)logical" when he has trouble translating such nuanced concepts into Federation Standard. Recall that this is a language which conflates (epistemic) rationality with (instrumental) rationality, requires lengthy explanation of foundational concepts such as curiosity, relinquishment, or lightness, and cannot safely dereference the
void*.Third, we might accept as the premise that Vulcans do have emotions, but (like humans) also some difficulty safely expressing them in ways acceptable to both Vulcan and most twentieth-century human cultures. It is therefore unsurprising that the sources where we tend to find evidence of Spock's richer expressions of emotion and deeper relationships are often disputed or dismissed by historians.
In summary, we can understand Spock as a competent officer disadvantaged by cultural and language barriers and unfairly maligned by the televised medium. I propose that we refer to this half-Vulcan, half-steelman as Spock′.