Art is subjective so I do not believe something needs to have a good ending or be endlessly perfect to a masterpiece. There are flaws in everything if you look for them and if 99.9% of the text is great nd that meh part is at the end I don't think the work has to be defined by that 00.1%.

I will always appreciate a swing and a miss that is bold in its effort than safe. I overall have great feelings toward the TV series but it got to safe near the end and lost any attempt at being bold. The books are still making some crazy swings. Fire and Blood and A World of Ice and Fire have some incredible world building and moments and AFFC and ADWD make a ton of bold and weird choices.

I feel while the the series is a masterpiece. The detail and depth is mindblowing. The fact that a character first mentioned in a seemingly throwaway line about a lord in a minor house who lived two hundred years prior can feel real, have a good depth of character and can logically impact the world to an amount feels right to their stature is impressive alone. The fact that are not dozens, but hundreds of similar characters is mindblowing. That's a mastercraft of worldbuilding.