I know that boxer engines are those which have pistons moving in the opposite direction.
Also the fact that since they sit low, the center of gravity is lower which causes better handling.
Boxer engines are large and complex that is their greatest fault.
The engines have a larger widths when compared to an inline 4 or a V6. If you have ever replaced spark plugs on a Subaru those engines are shoehorned in very tight. Not all manufacturers want to work around that kind of width.
The engines are also more complex. The cylinder block is split into two halves. This requires the manufacture of two complex items instead of just one.
Finally the needed orientation of the engine provides extra problems. These engines are oriented in a traditional manner. Because the transmission sticks out the back the power flow has to turn 180 degrees to go to the front wheels. Since most vehicles are front wheel drive this makes the transmission more complex.
The advantage of a boxer engine is that the engine counter balances itself. The pistons that are across from each other move toward the crank and away from the crank at the same time. This gives smoother operation and a crank shaft that is lighter because no counter balance weights are required. It also reduces vibration.
Another advantage is that because the engine is wide and short it lowers the vehicles center of gravity. This reduces body roll and improves handling.
As far as a complex transmission in concerned, Subaru uses it's complexity to its advantage by the addition of AWD.
Like V configuration piston engines , boxer or flat configuration engines can be made approximately half the length of a straight configuration engines. Compared to V engines boxers are not as tall, offering the ability to position mass lower in the vehicle's chassis, but are wider.
All boxer engines feature low primary vibration as pairs of pistons are opposed and move in a way to cancel out each other's momentum.
Like Vs boxers are more complex that straight engines, having 2 cylinder heads with associated head gaskets, exhaust manifolds etc., and double the number of camshafts, more complex camshaft chains or belts and more complex cooling.
Compared to Vs, boxer engines have a more complex crankshaft as they have a crank throw for each connecting rod. True V's have pairs of rods sharing each crank throw.
The probable reason boxer or flat configurations are not used very often in modern cars is because a straight or V engines are almost always a better choice.
Many cars are now front wheel drive which work best with a compact, transversely mounted engine. A straight-4 or a V6 are much more suitable that a flat-4 or flat-6 in this configuration.
Longitudinal mounted engines are preferred for front-engine, rear wheel drive, the boxer engine's lower mass probably does not provide enough advantage over the simpler straight-4 or V6.
Where boxer's really do seem to work is in air-cooled rear-engined cars (e.g., the VW Type-1 and Porsche 911) where width is less of a concern, the engine must be a short as possible to avoid a large mass behind the rear axle, and the banks of cylinders are easier to cool as they are more widely spaced from each other.
With regards to the modern Subaru layout, which originally featured 4-cylinder engines and all wheel drive, a longitudinal mounted boxer makes sense. To accommodate front wheel drive-shafts the engine must be placed in front of the front axle line, and the short, low boxer engine has advantages. (Compare this with the original all wheel drive Audi 80 Quatro that had a straight-5 hanging way-out ahead of the front axle.)
At one point flat-12 engines were used in racing cars such as the Porsche 917, and many Ferrari F-1 cars (although these were technically 180° V-12s, which may have any cylinder bank angle). Flat-12's proved incompatible with ground-effects as the cylinder heads occupied the space where the venturi tunnels needed to be.
I have a '15 forester with the 2.5/6m configuration.
Replacing the spark plugs is expensive but that is about it for any disadvantages. Oh, and it's a bit underpowered.
The flatness of the motor allows for a very low hood - which provides for a large windshield. Forward visibility is awesome. It also lowers the vehicle's center of gravity, making for excellent snow/rain cornering.
The flat engine primarily allows Subaru to utilize equal length drive shafts for both front wheels, resulting in quick AWD actuation for the price - notice I said "for the price."
Boxer engines do have their place and are especially suitable for conditions where high continuous torque is required and continuous cooling is offered. The newest cars with CVT transmissions are a great match for boxers, but currently aren't offered, because of transverse arrangement....but it's only a matter of time. Some of the most legendary race cars like the Porsche 917 used a boxer. Many legendary Cessna and Piper aircraft still employ boxers as their powerplant of choice. The Napier-Sabre 24 cylinder double boxer engine used by Hawker Typhoon & Tempest produced an astonishing 3,500 HP. Drivers who like to punch accelerator pedals and shift gears won't find boxers appealing and will remain confined to needing a computer to manage inline or V-engines and transmissions for them. Boxers are also a good match for hybrid cars where a constant-rev power plant recharges batteries to extend the cars range. Inlines and V-engines are for different conditions that compensate for traditionally abusive operating conditions.
