Nordschleife was a great road course that completely vanished (and the name should have with it) when F1 took over from GT racing and completely changed the course. Still, for the F1 cars its a pretty good test of speed and handling combined. (I'd still like to see them on a road course 5 times the length...that would be fun.
The others historically were odd shaped circles. Cars of this day probably wouldn't even have to apply brakes (much like Indianapolis, where so many road race enthusiasts bore themselves to tears every Memorial Day in the US). So I would say that modern times have made them a little more challenging than less.