What are some of the reasons some engines are more prone to head gasket failure? Is it the way coolant and oil passages are designed and distributed? E.g. Subaru is notorious for head gasket leaks.
A few of the main things I can think of....
Higher cylinder pressures put more strain on the combustion ring.
Detonation/pre-ignition/pinging, especially on high compression or forced induction engines, can blow out the combustion ring.
Warped cylinder head, typically due to overheating.
Close proximity ports. There isn't enough gasket material between the ports, making the gasket inherently weaker.
insufficient clamping force on the head - head bolts/studs.
I can comment on the subaru part of your question. Subaru stands by their 'boxer' engine type, however one flaw to having pistons firing horizontally is that gravity plays against you in the long run. When you think about pistons firing horizontally and rings wearing down, general wear and tear to intake and exhaust valves, coolant leaks, this puts strain on the now completely vertically mounted head gasket. This is why head gaskets fail with a higher incidence in the subaru boxer engines. http://kentbrothersautomotive.com/an-explanation-of-common-subaru-head-gasket-problems/