While not the OP, there is research demonstrating that the blood glucose and insulin response to various carbohydrate sources is highly specific to the individual. In the study I'm thinking of, some people were given cookies, others were given bananas (among other things). The responses weren't consistent at all.
As for calories in/out being a simplification, while it is mostly true, the tricky bit is that calories out actually varies wildly with the foods you eat. Your body's hormonal milieu changes very significantly with what you eat, and that has a massive impact on your biochemistry. Additionally, the microbes in your gut have certain food preferences, and they can metabolize a significant fraction of the calories you eat. Thus, hormonal shifts and rate of microbial metabolism can spike your "calories out" far above what would be predicted by BMR and activity level (and the reverse is unfortunately true as well).
