I'm pretty sure it's the corner loads with the stock wheels that cause these to break. With the stock wheels using washer lugs and being hub centric it is possible that they can move a fraction under high load, i.e. when pulling a 1G+ corner. This movement is getting transferred to the hub ring which simply is so weak that it fractures.

So far 100% of the failures are on front wheels that I'm aware of. On our cars the front hubs are under far greater stress than the rear and take the brunt of the car's weight under deceleration and turn in. I'm very careful about how I mount and dismount my wheels, I'm positive that it did not happen during that process.

In my case it might have been a combination of Spearfish's point 2 and Chris' point. After installing the spacers I have taken of my wheels a couple times for swaybar install and break pad swap.

I doubt uneven tightening of the lug nuts was a problem in my case as I always take extreme care of properly tightening the lug nuts. But taking the wheels of or putting them back on could have been part of the problem, I might have bumped the center ring, the wheels are big and heavy and therefore rather hard to handle. Ad to that the cornering and braking load of the 2 autox events and a track day and you have the perfect recipe for a pair of broken hubcentric spacer rings.

I'm done with spacers for now. To bad, as I think our cars (on stock wheels) do need
Spacers to complete the look...

Stupid question, but how do you keep the front wheel from spinning in order to loosen the lugnuts without initially loosening it while it's on the ground? Do you just have another person around to stay on the brake?

No issues here with both 370Z's installed with 20/25mm for over a year now with 10k miles on them. No AutoX nor Track days, just alot of canyon carving.

By using an impact wrench.