What's the difference between:
4340 steel in the US and any where else in the world?
A German cnc crank grinder located in the US or any where else in the world?
A forging press located in the US or any where else in the world?
What's the difference between a scat crank and a sonny? In my opinion, nothing strength wise. However, I strongly support US made products and because I have the means, I would buy a sonny crank over a scat. I've only seen one scat crank break, and it was the fault of the owner having the blower belt too tight. So for someone on a budget, building a motor, do I think a "Chinese" rotating assembly is okay? Absolutely! But probably not for anything over 600-700 hp.
Now, what cranks, rods, blocks, etc is mercury using that makes their product better than the small engine builders? NOTHING!
You are paying for insurance, warranty, the name mercury, etc.
You could argue this crap for days. My favorite term to hear in the sales world for racing shit is " aircraft grade aluminum" I laugh my ass off ever time I hear that.