It has not always worked well. The first US government structure, the Articles of Confederation, lasted only about eight years. Since 1789, when the US Constitution came into force, there were notable incidents of governmental problems, including the inability to deal with slavery that led to a bloody Civil War, some of whose hard-won gains were reversed with the end of the Reconstruction. Other bad decisions can easily be found, such as foreign policy mistakes that led to entanglements in Vietnam and Iraq and/or made such entanglements more costly. Members of various minorities through history have often had difficulty exercising the political and civil rights that they were supposed to have. A tendency to spend faster than tax revenue means increasing budget deficits and national debt. Today, everyone seemingly agrees that medical care is an expensive mess in the US, but there is so little agreement on a solution that any change will likely be corrupted by having to get buy-in from numerous interests and ideologies.
Of course, this does not mean that dictatorship or one-party state is generally better, since most such governments govern worse and more oppressively for most of the people who live there.