The biggest drawback in series connection results from speaker tolerances. Look at a typical impedance curve and you can imagine what happens, if two coils form a voltage divider, when the impedance is not exactly matched at each frequency. The frequency response of each speaker will become uneven. If you are lucky and the speakers are not totally different, they add up to an even response again.
The efficiency is not changed, because you need the same amount of power for the same SPL. The SPL for a given voltage is lower, because less current flows, so you need to adapt your crossover for a linear frequency response over the voltage.
