The word "inferior" is probably a little too strong, but there is no denying that Japanese vehicles are of higher quality than American cars.
Japanese cars last longer and retain higher resale value than American cars. After the first 100,000 miles, Japanese engines show very little wear and have lost less compression than American engines.
A key element is attitude toward "manufacturing tolerances." The Japanese demand much closer tolerances and better fit than Detroit, which results in higher quality mechanisms. American manufacturers are content to have a "looser" fit for easier (and lower cost) construction.
It is also no secret that Japanese cars cost more than American. This really is, in part, due to the fact that it does cost more to design and build a Japanese car, but the car does last longer. So, if it should need repair, it is worth it to invest the money to fix it, because the repair would still be less than the depreciated value of the car.
By keeping their products lower priced, American firms create a dilemna when the car needs major repair. After factoring in depreciation, the owner often determines that he may as well spend the same amount of money on a new car. This pushes more sales, which is all American manufacturers are concerned with anyway.