I don't want to sound like a broken record but I say it's because it's GT4's physics. I don't have any problems with it in FM2.
It's kind of hard to blame it on GT4's physics, and then bring in the disgrace that is the FM2 'Ring.
For me, the 'Ring isn't so much hard, but a challenge. The 'Ring is very easy depending on the vehicle. Many slower type cars like a Civic will find the Nurburgring just a long drive. But for cars like Porsche, the challenge and difficulty comes from driving fast. The main reason I don't find the 'Ring very hard is because after many, many laps on various games in various cars, and a few laps in real life, I can easily imagine the layout. The only thing difficult to me is finding the ability to go very fast through the course.
And the way I've done so is by finding the 'Ring's "Roller Coaster Line". Confusing, yes, but to me, there is a certain line throughout the course that will make any car follow the "smooth" movements of a roller coaster. This is highly thanks to the elevation you encounter on the track mixed with the different sets of turns. The best area I find to find this line is by driving from the Karussell to the Pflanzgarten II. A better area to really see it in action is starting at Hohe Acht, and running to Eiskurve. Just running in this area should show you that "roller coaster" feel of the track.
In explination, it starts something like this. After leaving Wippermann, you'll begin running to a left hander that'll appear to have a bank on its inside corner. The bank is incredibly small, but is enlarged in feel thanks to coming off the hill before it. Afterwards, you'll very soon, come off a right hander that dips into a short piece of straight. As soon as you come off the banked-down curve on to the straight, you'll be climbing back up. Right here you should be getting through these turns at quite a speed. But it will require a short amount of braking as you begin an uphill, right hander that leads into Eiskurve. Afterwards, you'll encounter Pflanzgarten I and its turns. This "feel" continues on til' you get to Schwalbenschwanz. Thanks to this chunk of the track, imho, you can really see the smoothness of the line as you feel the car's body going from side-to-side by the turns as well as it dipping in the front and back thanks to the elevation. Couple this feeling together, and you'll get something very similar to a roller coaster.
The big difference between the coaster though (despite there being no rails on the Nord.), are the bumps. This is what makes driving the line tough as nails. One second, you'll be running that line, the next, a surprise bump could throw the car off it. Now, add the bumps, the line, and the type of vehicle, and you got yourself a pretty challenging combination to speed through.
Now, I don't know the course entirely by heart, with every little bump, and every little angle of turn, but I do know enough from experiences to know how the track runs and what can be done to ensure a great time on it. As said earlier, the type of vehicle plays such a huge roll in how hard you can push on the track's racing line. It felt amazingly different to run the course in a Mini Cooper S and go to a Ferrari 308 GTS the next day. Different cars, yes, but also a different experience.
My theory on the course is, the faster your car will take you, the bigger the difference you'll see in fear and excitement. It can be a laughable, exciting moment in the Mini 1 lap. And a terrifying, craptastic, intoxicating, yet utterly unreal, amazing ride the next in a Ferrari, Porsche, or any other car that's built for speed.