This test is valid for cohesionless soil because it measures the immediate settlement only which can be achieved in a short duration. Hence, It is called short duration test.
Limitations of plate load test (PLT):
It is essentially a test of short duration. For clayey soil, it does not give an ultimate settlement. The load-settlement curve is not truly representative.
The test on a plate of size larger than 0.6 m width is difficult. 
The test should be placed at the water table level if it is within 1m below the footing. If the water table is above the footing, it has to be lowered by pumping. 
The ultimate bearing capacity of clays is independent of the size of the plate, while for cohesionless soil it increases with the size of the plate. 
The plate load test does not truly represent the actual conditions if the soil is not homogenous and isotropic to a large depth.