Advantages:
- Lower center of gravity (most of the weight is lower down but the oil pan and exhaust can prevent it from being lower vs. a V engine with dry-sump lubrication)
- Better balance which means no balance shafts or as many balancing weights on the crank which means more efficiency and smoother feel (good primary balance and natural firing order for a 4 cylinder but it still has some secondary imbalances as a 4 cylinder - this is a bigger advantage over 2.0L as inline 4 engines really start to suffer without a balance shaft)
- Shorter than inline engines so you can mount it longitudinally in an all wheel drive vehicle without much overhang in front of the axle
- More room above it for a lower hood or a top mount intercooler if desired (which results in less turbo lag)
- Better self cooling ability since it's more open/spread out
Disadvantages:
- Wide (less room for double wishbone suspension or other components and may be limited on stroke)
- Cannot be placed as far back in front as an inline engine with the FR layout because it gets in the way of the steering rack which means less than ideal front/rear chassis balance (for grip)
- 2 sets of cams and timing chains kind of offsets the lack of balancing weights vs. inline engine (this is less of a drawback/waste for 6 cylinder boxers)
- Not as easy to replace spark plugs vs. an inline engine
Overall though, I prefer boxer engines and think it makes the 86/FR-S/BRZ that much more special.
