Phase rule deals with system in equilibrium and is not of much help for the systems which attain equilibrium slowly.
Since no quantitative analysis is done, it is necessary to determine exactly the number of phases present under the equilibrium conditions.
Though each solid is supposed to constitute a single phase, phase rule cannot be applied to solids in finely divided state.
It does not furnish enough information regarding the extent of changes when the system shifts from one equilibrium to another.
It conditions that all phases of the system must be present simultaneously, under the identical conditions of temperature and pressure.