I would say the Cons is mainly that you are kinda weak early and abit hard to farm with but over all there isn't much that is lacking in this build.
For Pros.. wow this build helped me climb after the s4 reset. It got so nice utility and sustain that you just wanna jump in to the game and hug him.

The people i've been playing with this almost told me the same thing after every game, win or loss. "Nice Position"
And I feel that those two words means alot in League, if you got the position to be you can easy snipe down everyone wtih Last Whisper. Remember that guys POSITIONS WINS!