Part of the whole thing about going flat is to rid yourself of unnecessary decor. This also means moving away from extreme levels of detail in favor of big, bold symbology - whittle down to the basic symbol and use that.

I think they've done this well for NYC and SF - large swathes of color, immediately recognizable symbology, and large undetailed shapes that are still recognizable.

Ditto Japan, though that one sort of derived itself.

Berlin, London, and Shanghai are weird though. Berlin and Shanghai especially - highly detailed silhouettes on what is otherwise a very spartan backdrop. Seems incongruous, especially when paired with the first two icons in the set.