The book convinced me that even in the 18th century, it was already the case that an imperial power could not enforce its will on a faraway land whose inhabitants were determined not to ruled from abroad and who were fortunate enough to be reasonably well equipped and led. What Stephen Walt wrote recently about modern U.S. failures in counterinsurgency wars (Vietnam, Afghanistan, Iraq) is not really different from what the British were shocked to encounter on the American continent n 1776.
So the Brits learned despite their best efforts between 1775 and 1783. The war did cost the colonies the lives of one percent of their populations, something we easily forget at over 200 years distance.
I also was reminded by McCullough's narrative how fortunate the emerging American colonial project was that its political representatives put their trust in a general whose role model was the Roman dictator Cincinnatus. (Here I'm presenting my own thinking, not McCullough's in this book.) After leading the Romans to victory, Cincinnatus distinguished himself by resigning his authority and returning to his farm, thereby becoming the Roman ideal of virtuous leadership.
Whatever his other gifts (McCullough enumerates many) George Washington seems to have been extremely sensitive to keeping the good opinion of those he thought to be the right sort of people. And that meant he seems never to have considered using his unique national esteem to become a Napoleon. Our ancestors got very lucky.