I also believe cracks are random. I have a firm belief it's related to the mold process used by some of the injection mold vendors that DJI uses, or cheap materials, or a combination of both. Plastic injection molding is a science. I have a customer that make injection mold machines (Husky), they have told me lots of stories of how injection molding can go bad just in the process, temperatures, materials, timing and cool cycles. If you don't take the science seriously the formula can turn out crap that may look OK on the surface, but the integrity is brittle garbage.
It would be interesting to know how many vendors DJI buys their injection molded shells from. I haven't heard of anyone noting any kind of ID marks on the shells that might indicate who made them, or a code so DJI can ID who the maker is of the cracked shell so they eliminate that bad vendor. It's my "guess" that DJI simply has a bad injection mold vendor, and DJI doesn't know how to vet a new injection mold vendor and their process before they engage in business.
I bought the Strong Arms as protection against catastrophic failure with a motor dismounting itself during flight, which was reported. That story kinda made me pause, I decided $25 was pretty good insurance against that from occurring. It's obvious to me that using Strong Arm brackets spreads the pressure of the tighten screws over a larger area, relieving the specific areas that have failed around the screw cavity barrel areas. For those that use Strong Arm kits that have compared the twist flex of the motor pods, with and without the strong arms installed, it's clear there is less flex in the motor pod with the kits installed. They feel stronger.
Did the Strong Arms prevent me from getting shell cracks? I don't know, I never will, but I can say I don't have any cracks. Am I'm glad I bought them? Yes, it doesn't hurt to have some extra strength and insurance.