True but GOT is not Sci. fi., to be honest, I don't really know what it is.
Whether it's sci-fi or Fantasy is completely irrelevant. The same principles of storytelling apply. And the problems with the later seasons of GoT weren't the fantasy elements, but that established characterizations, plot sturcture and often logic were thrown out of the window to make some sort of fanfic that only existed to deliver "shocking" twists and "kewl" one liners for the trailers.
So the problems were independent of the setting and chiefly originated from bad writing.