Something else to note about a "flat" configuration as opposed to the other types:
The engine "case" as opposed to a "block" supports the opposing dynamics of the pistons, contrary to an engine block, which requires huge "mains" to keep the whole setup from pushing itself out the bottom.
By the way, Subaru also uses the flat configuration on their 4-cylinder vehicles...
I found this to be an interesting topic, given that all of my vehicles each have a different engine configuration...
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car