The original book is narrated by the horse, who changes in the original story from an innocent colt to a survivor. But the film chose (for understandable reasons) not to be narrated from the horse's point of view. This transformed it from being a linear character journey into a circular story, a series of episodes before finally coming full circle and reuniting the horse and his first owner.
This meant the human characters had little opportunity for development. The chosen solution was to bring on the violins and dollop out the sweetness. Most of the characters Joey the horse encountered were 'good' - incredibly good. There was no shade to their radiant goodness. The other non-good characters were, surprise surprise, utterly bad. There was no light to their brutish badness.
We all know this to be untrue. People are never wholly good, or wholly bad. They are multi-faceted, and fictional characters need to reflect this. And if ever there was a time when characters are tested it's war. Essentially good people are put in positions when they are compelled to behave badly or selfishly. People who have got into the habit of behaving brutally find a chink of humanity.
Ambiguity in characters is eternally interesting because a) we don't know which way they'll go next and b) it's the truth. Smothering characters in golden syrup is no substitute for character development.