Lightweight and compact, SUBARU BOXER Horizontally-Opposed Engines are intrinsically smoother and more efficient than In-line or V-type designs. Superb rotational balance means less vibration, because the pistons counteract each other to cancel it out. Obviously, the flatter configuration also means the engine sits much further down in the vehicle, giving a lower centre of gravity that enhances the advantages of the AWD and suspension systems.
This layout is perfectly sound according to engineering principles. Balance is good, but in practice, the upward and downward movements of the pistons in this design create unwanted vibrations and a lack of rigidity.
The idea is correct in theory: make the engine compact and you should also help counteract the tendency for pistons to fight against each other. However, in high-rev, high-torque power bands, this design still creates unwanted vibrations.