折减法和超载法都有适合各自计算（即接近客观实际）的工程条件（如工况和边界条件），不同的工程条件应该用不同的合理的方法。如果说是折减法是错误的，那我认为是最大的错误 - I agree to tthis,
Most countries adopt the concept of strength reduction, but in France, the second method 超载法 is also used. Because the problem is nonlinear by nature, the two results are not the same.
Be careful, FOS depends on definition. and both definitions can be adopted. For slope failure of natural slope arising from rainfall without any external loading, the first definition can be adopted. For failures arising from external load, the second definition can be adopted. Please also note that we can define FS according to height of slope, weight of soil, earthquake and other factors ! That means, factor of safety depends on definition of FOS !
I suggest we have to be more rational in looking at this problem. Some people here just critize the classical method without a throughout consideration.
We can also argue on different factor of safety to c and tanΦ, different factor of safety at different location, different factor of safety on moment or force, use of residual strength at part of the failure surface and the definition of active/passive factor of safety (i.e. FS=A/(B-C) or FS=((A+C)/B). See a report by Instution of Structural Engineers .in UK and some papers in Geotechnique.
Be more rational, we are arguing on different definition of factor of safety, and both may be acceptable depending on condition.