from someone who is a political scientist:
“Your questioner mixes two questions– parliamentarism vs. presidentialism and federalism vs. unitary states.
“Federalism has been adopted in many successful ongoing constitutional democracies, including Canada, Australia, Germany, India, and Spain.
“What hasn’t been adopted successfully is presidentialism. This is [a result of] both path dependence and selection effects.
“1a. Path dependence: Britain is parliamentary, and lots of the constitutional democracies in the world are former British colonies. Strong royal governors who existed in the 13 colonies in 1776 (standing in for a still-strong Crown at home), and strong republican governors filled their shoes, with a strong independent president following later. But by the time Canada, Australia, New Zealand, India, etc., framed their governments, their local administration and the Westminster system in London were parliamentary.
“1b. Path dependence: The [West] German Basic Law has been more influential and more widely-copied in the postwar world than has the US Constitution. And the fact that the U.S. planted parliamentary systems in Germany and Japan probably helped to kill off the thought that even the U.S. thought a separately elected strong president was necessary for constitutional democracies.
“2. Selection effects. Lots of countries have *tried* independently elected strong presidents. And they haven’t tended to remain constitutional democracies under that system. The U.S. political culture and underlying political conditions are very robustly republican-democratic-liberal; we could get a lot of institutional things wrong and still end up with a constitutional democracy. But where those things are more fragile, presidents seem to tend to become strongmen and dictators. Presidentialism has been a terrible failure in Latin America when it’s been tried– and it often was, in the 19th century, when the new Latin American republics took on the U.S. Constituion as a model.
“I’m sentimentally attached to presidentialism, and I theoretically like the stronger separation of powers you get with an independently elected executive. But the evidence suggests that the U.S. is unusual in being able to tolerate presidentialism and remain a democracy, and that parliamentarism is much the better bet for new constitutional democracies.
“(But parliamentarism is fully compatible with federalism–Canada, Australia, India, Spain.)”