restrictions of Miller's theorem

Wat r the restrictions of the miller's theorem usage.Can it be used in any feedback amplfiers to find out the Vout/Vin expression..will the expression derived with the miller capacitances in place give the same result without implementing the miller effect and findindg out the transfer function

Applying Miller's theorem always gives you an equivalent circuit.
You never have problems with this circuit if you do not change the excitation source.
If you do change the excitation source then you have to reapply Miller's theorem for the new conditions and you will get a different equivalent circuit.

Just two examples where Miller theorem is used not correctly.
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1. You excite the circuit at its input with a source Vs and get an equivalent circuit.
2. You then use this equivalent circuit to find Ro=Vt/It by connecting a
test source Vt to the circuit output and suppressing Vs.
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1. You excite the circuit at its input with a source Vs and get an equivalent circuit.
2. You then use this equivalent circuit to find return ratio.