There are no facts which point toward prop balancing doing any good with the DJI factory props. It's like prayer - lots of people will tell you it works. One religious billionaire commissioned a study - and it didn't work.
BUT, being a mechanic myself I do know that when I spend some time on a process - I feel good about the piece of equipment. It's like getting an oil change in your car - even if you didn't need it you somehow think the car is running better and thanking you for the attention!
IMHO - and I do this (check this industry) for a living, the shell problem is somewhere between 2% and 10%. But even those figures don't mean anything because it's more a matter of the ## of hours flown and the conditions that were flown in. It also could related to rough handling of the birds in packing and shipping and other things. Many users may have cracks and never notice them - etc.
But, again IMHO, I suspect the shell design was "just good enough" and that the vendors and/or tooling (or formula/heat, etc. used to form) were responsible for some bad runs.
Also, the P3 - being so much more capable than former models - tends to get used a lot more. It works better in wind and other conditions so it's subjected to more hours of flight time and more stresses.
I've owned a lot of Phantoms and you won't find me ever balancing a prop.