If the optimized part is cracked, will it result in a total failure? What parts should and shouldn't be optimized? For instance, would you optimize a connecting rod? I have a few opinions I want to share, what are yours?
A lot will come down to how the part will be produced.
Connecting rods are likely cast, then machined. Or simply machined from stock.
Optimizing a connecting rod would likely lead to the need to create it via 3D printing, which is going to be more expensive to produce.
Other factors to consider are the trade offs in saving weight in a component like a connecting rod. Like a flywheel, a lighter connecting rod may allow for quicker acceleration, but it would also loose momentum quickly.
Regarding strength, I feel that optimized/organic parts can be stronger than their billet counterparts, but it really depends on the ability to capture all loads which will be seen by the part. Connecting rods are a pretty good example since they are largely a reciprocating component.
If money were not a limiting factor, and making a lightweight engine were the top priority, this might work out well.
I wonder if optimized parts shrink and swell at the same rate as solid cast or billet parts?
Durability would have to be assessed but then, you're going to assess safety concerns in any design. Some cracks don't lead to failure or that failure may not be catastrophic --no potential for loss of life. In some examples, it may only be a matter of setting good service/replacement intervals into the maintenance schedule.
All of this comes down to cost...
Most of a connecting rod's rotational energy is stored in the crankshaft/flywheel assembly but exploring connecting rods is a good way to think about this...
Why are aluminum connecting rods often used in drag racing?
Lightweight piston, rod, pin and ring assemblies have definite advantages there in terms of acceleration.
What are the replacement intervals?
Some series replace the assembly every trip down the track and others will replace at some interval due to fatigue concerns.
However, you'll never see an aluminum rod in an endurance race application. And therein lie some of your answers.