The Opposed cylinder design for gasoline engines is rather uncommon, though several have been made in the past.
The expansion forces in an opposed cylinder engine are balanced. For ever force there must be an equal and opposite force. Thus V shaped cylindar arrangements create forces that cause the engine block to vibrate. This seems like waste of energy.
Napier Deltic engines, a type of opposed cylindar engine, are described as "high strung" and require more maintenance. I would expect the same of any opposed cylinder design due to the use of gears instead of a single crankshaft. Ignoring the maintainence issue, is the opposed cylindar design more efficient at burning fuel?