Crankshafts have always struck me as being extremely difficult to manufacture, tho I guess they have had it figured out for a long time.
I do recall a historical restoration of a Messerschmidt ME-109. They found a crashed one buried in the ground in England. Fortunately they engine was relatively undamaged.
Rolls Royce agree to rebuild it but warned the people who recovered the engine to be extremely careful of the crankshaft as they would never be able to replace it.
The balancing has to be very good and there are oil passages needed to supply oil to the connecting rod bearings.
When Buick came replaced their aluminum V8 and V6 (selling that 215 cu in V8 to British Leyland was one of the dumbest moves on GM’s part) with the 300 cu in V8, the V6 was basically the same, the obvious difference was the 2 rear cylinders lopped off. Both engines shared piston and valve assemblies, but the V6 crankshaft needed to be balanced differently because of vibration issues specific to a V6 design.
Don't know why. Todays metallurgy is more advanced. Modern CNC offset grinding could reproduce virtually anything.
As a lifelong (retired now) machinist I can tell you the short answer to making CS's. You turn them like a CS as you're cutting (turning). Everything starts out as round and true as you can make it. It's not as difficult as it seems when you take it step by step but it is very advanced machining.
There was a movie that came out in ‘69 about the Battle of Britain.
The ME-109s in it were borrowed from the Spanish AF.
Amusingly, the engines in them were Merlins.