To what extent do the separation of powers and checks and balances limit government in the USA?

The question asks to assess the effect to which both the separation of power and the checks and balances limit the whole government of the USA, not just its executive branch. Firstly, the separation of power assures that each of the three branches of the government consists of different people, so that one cannot be a member of two branches or have an authority over a member of the other branch. It assures that a small group of people should not be able to access all streams of power. This idea comes from the works of 18^th century French philosopher Montesquieu who argued in the “Spirit of Laws” for three branches of government as three separate entities that control each another. This is not a perfect separation, as the checks and balances imply some power of one branch on another and institutions such as the role of the Vice-President as the President of the Senate are limitations in extreme situations. Secondly, the checks and balances are overlapping powers of one branch on the another, when one branch actively limits the power of another. The main checks in the government of the USA are that Congress has the power to impeach the members of the executive, including the President and controls his budget. President can veto laws and pass executive orders, as well as nominate the Supreme Court justices. And the Supreme Court has, since the landmark case of Marbury vs Madison the authority of judicial review, the power to declare any law or act of executive branch unconstitutional. In reality, these balances work in many cases to the relative inefficiency of the government of the USA. Most notably, the gridlock of the Congress in the second term of President Obama was a great example of divided government, when all three branches executed their powers to limit the power of the other. Vetoes and executive actions on the side of the president, but also unwillingness to pass laws on the side of the Congress and appoint President´s appointees, most notably Justice Garland to the Supreme Court are a great example of the efficiency of these institutions. On the other hand, in the cases of unified government, such as at the beginnings of the terms of past four presidents, there is a good case to be made for limited extent of the checks and balances. However, as the present development under President Trump demonstrates, even in the case of unified government, the checks and balances can work effectively.