My son at age 11 or something did very well on chocolate milk. We thought that was the bottle food. Then he did a hot ride. Without thinking poured it over his head. That was the last of that.
Like most things, it depends how stable the conditions and rider are. You can do about anything if reality matches your planning. Just that for some, the surprises are part of the whole cycling adventure. I don't think much anything beats stuff in the pockets and water in the bottles for maximizing your options. I would not argue that at x degrees for y time at z effort the ideal formula would be something mixed into the bottle. Just that x, y and z are rarely what was expected.