Yes, they are resistant to swings based on mood changes in the population. However, this does not hold for changes of mind or strategy by elected people themselves. They might promise something at some point and then not do it. US history is full of broken promises.

A counterpoint is that Trump and Obama are elected directly and specially on foreign relations they have a huge power to do whatever they like. Obama promised some stuff, Trump retracted it.

> In contrast, a dictatorship is unreliable, because all decisions are made by one person, whose mood swings can result in lots of unpredictability.

That is true in theory but in practice many dictatorships are remarkably stable. A dictator has to please his cronies, he can't do whatever he likes. He can do what he likes as long as it does not touch the interest of the cronies.

Check "Selectorate theory" in political science. Or this nice little video: https://www.youtube.com/watch?v=rStL7niR7gs

Representative democracies are notoriously prone to populist shifts; the US, though, has a system with many different and conceptually independent levers of power checking each other which is generally thought to make it resistant to populist shifts, and this was probably quite true in practice before the era of modern mass communication and nationalization of most politics and political movements when those levers really were largely independent in practice as well as in theory. But it's not the late 18th Century any more.