There are many reasons why the Subaru engine is superior to the vanagon's original engine and why it is the most popular engine conversion. The following is what we think are the most important points:
Subaru uses a modern fuel injection system, which eliminates the need for any adjustments to the air/fuel mixture and timing. It also automatically compensates for altitude changes and uses a knock sensor to help eliminate possible engine damage from using poor quality fuel by adjusting ignition timing. The cooling system design is much simpler and far more reliable than the waterboxers huge array of special hoses, plastic bits and poor cylinder head gasket design all of which are prone to failure.
Ease of Maintenance
The accessory drive belts are of a multi-rib, serpentine type, which last a long time and require minimal attention compared to the convoluted v-belt setup that VW used for its accessories, which need adjustment often and have problems with cracked alternator mounting brackets. Service intervals are longer than the VW engine as well. for tune-ups, and the most involved service item, the timing belt and associated parts, has a 90k mile replacement interval and is very reliable. Parts are easy to source in most places as well.
Added power, torque, and fuel efficiency
EJ22 2.2L engines make between 130-145 HP and ft/lbs of torque, EJ25 2.5L engines make between 165-177HP and ft/lbs of torque. Fuel mileage goes up slightly over stock, typically 1-4 mpg depending on driving conditions, style, van type, etc. These engines run perfect with no issues on midgrade fuel at all altitudes.