One of the major advantages of the British army was that it was one of the most powerful and experienced armies in the world. During the previous 100 years, the British army had defeated many powerful countries in war, such as France and Spain, and seemed almost unbeatable.
The Patriots' advantages included.
Fighting on their own ground.
A higher sense of purpose and motivation since they were fighting for the freedom of their own land, which gave them an advantage over the hired Hessians of the British army.