What is the Voltage output of this Unity gain amplifier?
Nothing has been said about the open-loop gain of the opamp.
Is it very large? What about a frequency which allows an open-loop gain magnitude of only |Ao|=10 ?  

In this case, we cannot neglect Ao and we have not Vout=Vin. 

Instead you have to use the classical formula

Vout/Vin=Ao/(1+Ao) with Ao=f(jw).