Actually, based on the cross-section, it looks surprisingly simple for what it's doing. And it looks like it was quite compact for its power output at the time (before turbos were common). Weaknesses probably included lubrication for the upper crankshaft, cooling, and power coupling from the upper to the lower crankshaft given that most of the power is taken off the lower. Plus shop issues with things like having to remove the top crank for access to the power assemblies.

That Deltic OTOH is just nuts. It's the kind of thing that only the English could design and only the Japanese (who of course were not involved) could make reliable.