It’s what the US Framers referred to as a “parchment guarantee”, nothing more than words on paper.
The actual structures of power have changed quite dramatically roughly every 70 years in the US, from getting rid of the idea of states as sovereign with the Civil War to gutting the interstate commerce clause of all meaning under FDR or there President arrogating all war making powers to himself against the quite explicit provisions in the constitution. Most recent huge change would be either the administrative state after WW2 or getting rid of freedom of association.
There are plenty of countries that have the same nominal separation of powers, guaranteed by a constitution and a Supreme Court, but where the nominal separation of powers matters little in the face of political games. Hungary and Poland are relatively well known recent examples of countries that have subverted strong constitutional guarantees by inventing exceptions or finding reasons to oust people who are opposing political will for institutional reasons. In more corrupt states it can happen more egregiously.
A big problem, especially in young states, is the natural tendency to dislike red tape. Institutional power is all about red tape, though - doing something through the proper channels, with the proper oversight, even if it takes 3 times as long and costs twice as much as it should. This is the biggest hurdle that a young democracy has to overcome - learning to trust the apparent inefficiency of complex institutions (and knowing how to reform institutions that really don't work).