Disadvantages of direct shear test: 
The stress conditions across the soil sample are very complex. The distribution of normal stresses and shearing stresses over the potential surface of sliding is not uniform. The stress is more at the edges and less in the centre. Due to this, there is progressive failure of the specimen i.e., the entire strength of the soil is not mobilised simultaneously.  
As the test progresses, the area under shear gradually decreases. The corrected area (A) at failure should be used in determining the values of σ and τ
As compared to the Triaxial test, there is little control on the drainage of the soil

The plane of shear failure is predetermined, Which may not be the weakest plane
There is the effect of lateral restraint by the side walls of the shear box.