The presence of sensitive clay pose a challenge when performing slope stability assessments. Because of the strain softening behavior, the validity of conventional calculation methods based on the principle of limit equilibrium (LE) are not fully valid. This paper studies downward progressive failure in long natural slopes with an aim of identifying the governing parameters and the validity range of LE methods. A FEM approach which accounts for the non-linear stress-strain curve of the material, including the post peak softening behavior, is used. Sensitivity of the analyses to variations of key parameters like in-situ shear stress at the failure plane, brittleness, stiffness of the soil mass, and geometry are investigated in terms of critical load for initiating the slide and the corresponding critical length. The results show that the capacity of the slope in terms of external actions is reduced the steeper the slope is, the more strain softening behavior the material display and the lower stiffness the overlying soil has. The initial shear stress level is identified as a highly sensitive parameter. Further, by studying variations in the critical length it is indicated that the validity of classical LE methods is limited for steep slopes in soft and very sensitive clay.