Its very difficult to pass crash standards with such a long engine. Basically the engine wont crumple so it reduces the space for crumple zones in the car.
The Nissan GT-R is a very precise driving tool. They measured every small bit of it and tuned it for optimal performance. Straight six cylinders mess around with the center of gravity of a car because they cant be fitted behind the front axles since they are too long. Therefore they sit on the front wheels and make the car nose heavy which makes it understreer in tight corners.