Unsubstantiated?
How about claiming a cast iron crank can't handle 300 HP?
I'm sure the crank would have trouble with prolonged detonation but if I'm right and the engine can handle boost at high rpm without detonating, the crank is one of the last things I would worry about.
Most crank failures are actually caused by something else that's wrong, connecting rod failure, too little bearing clearance, main caps walking, block flex, and high loads on the snout (supercharged).
The way modern aluminum engine blocks are designed for NVH and the durability testing that is routinely done during development tell me that the connecting rods are the most likely point of failure because OEMs usually use small cheap bolts and sintered powdered metal cracked cap rods but computer simulation and design have come to a point where Ford can get the Coyote engine right on the first shot on a compressed schedule with more than twice the power, a higher rev limit, longer stroke, wider main journal spacing and 90° throws which all require a stronger design than a little short stroke flat crank engine with a 6,350 rpm red line.
Claiming that Ford would widely release an engine without enough design margin to handle a modest increase in power is silly.
Turbochargers on the other hand have a rev limit where compressor blades can shear off due to the forces. The only way to build boost is to force more air in to the engine and when you increase boost you have to increase the turbo rpm. When you get to the limit you have to do things like reduce boost to control the speed. Ford would do that because they were optimizing for low speed torque and fast spooling.
Looking at the Cobb tunes, it looks like that is exactly what they did.
Barring any evidence like a broken crank or someone that can get a stock turbo to build more boost at high rpm (it hasn't been done), I've got to draw a conclusion from what is out there.
My "unsubstantiated speculation" is actually a pretty educated guess along with being told that the turbo is too small by one of the guys from Cobb.
If it looks like a duck, swims like a duck and quacks like a duck, its probably not a kangaroo.