Bad jugs...
While I can't disagree with the fact that piston aero engines are somewhat primitive, there's one great advantage in their design, at least in the case of horizontally opposed engines and, in particular, the O 360. Sometime ago we had to pull a bad cylinder in an HIO 360 pistonbanger in a Hughes 300. It was absolutely a snap to remove and reinstal after the engine shop rebuilt it - all in about a day.