E=F/Q is always valid - for non-uniform and uniform fields. It is the definition of electric field - force exerted on unit positive charge. The only difference is that in a uniform field it will be the same everywhere, as the force on a unit positive charge will be the same everywhere, whilst in a non-uniform field it will vary in space, as the force on unit positive charge will depend upon the location of that unit positive charge.
E=V/d is the electric field at any point between two parallel plates, where V is the potential difference between the plates and d is the separation of the plates. It is only valid for the uniform field between two parallel plates.