As Marin noted, engine failure in the majority of circumstances happens because of things attached to the basic engine. Many, if not most, of those kinds of things are fixable with a basic spare parts inventory on board, i.e. belts, filters, hoses, impellers, and a bit of knowledge to clear filters, air locks and the like. In today's diesels, catastrophic engine failure is an incredibly rare event. Thus, in my mind, the single v twin arguments has devolved to being mostly a personal preference issue rather than a truly pragmatic one. That said, I have always preferred twins on the quaint notion of having a built-in "get home" system that requires no action from me whatsoever. I have yet to find any other "get home" that convinces me that it is functional and effective for any significant distance in any significant sea conditions, whether it be the Nordhavn folding sail prop driven by a 62HP diesel in a 120,000 lb boat or a hydraulic system driven off a 20KW generator. My personal comfort level is twins with shafts and props fully protected despite the addditional up front and operating costs.
But, hey, that is just me.
But, hey, that is just me.