Casings fail in a brittle fashion, forgings in a ductile fashion, in general.
The difference is not in strength or stiffness. The affected property is "fracture toughness". It is the ability of the material to absorb shock loads without damage, and forgings *always* outperform castings.
Don't even bother bringing up nodular cast iron, AKA ductile iron. compared to a steel forging, it is still a weak sister.
Fact:
Forging is better, casting is cheaper.
Also fact:
You will never know the difference between a forging with a factor of safety of eight, and a casting with a factor of safety of five.
The BHP frames warped because they were poorly struck, and improperly heat treated (normalized) before finish machining. Nothing more.