There's also the not-insignificant issue of harmonics. Where I work we tried building our own twin-crank Diesel, and after several million on developement, we shelved it due to harmonics causing havoc on the crank and bearings. Just because something can be bolted together with existing parts doesn't guarantee no issues. Just sayin'.
Funny you should mention that - we were lightening F4i cranks for FSAE cars and ran into similar issues.
The company doing the work claimed to have many successful drag and sport bike people using their services with no issues, yet every engine we had with a lightened crank would always fail at the #3 rod and #3 main bearing. At first we blamed it on oil starvation but even after ensuring oil pressure was never dropping below 40 psi, we were still trashing engines.
We would usually replace the good engine with some pile-o-crap engine that was built using a honed block (thanks to scoring from a previous failure) and parts from other blown up engines. Since the cost of the engine was usually the cost of the new head gasket and bearings, no one ever took it easy on them. They got pounded relentlessly yet they never failed.
We were never smart enough to figure out the analysis but after reading several crankshaft design books, we decided that removing the weight moved the natural frequency of the crankshaft into the engine's operating range when installed in an FSAE car. The rev limiter probably didn't help help to prevent any weird harmonics from forming either.
Lesson learned: professional engine designers >> a guy in a garage with some tools