The second category is riders who are more ambitious than they are experienced or intelligent. Nothing more needs to be said here.
IMO the optimal age for a future international horse to be going Advanced is 9. You can do an Advanced late in their 8 year-old year, but really, the focus should be on confirming that they know their job on XC. If they've been brought along properly and they have the right brain for the sport, they should be able to contend with anything at this point. There should not be re-starts and try-agains, which we've seen in recent years with some very high-profile horses.
I breed only for eventing and only for the type of horse that I like. It's a personal endeavor. The OTTBs I've picked up are more for a general audience, and in general, they're easier to work with and train, and, as my homebreds are very slow to mature, the OTTBs make faster progress.
(There was a nice post yesterday at Eventing Nation on the Biddesden Stud, the Guinness's breeding program that produces Tamarillo. Yes, they have loads of money, but they also have patience and persistence in developing a successful yet non-traditional program.)
As a final point, I had no idea the USEA still published a magazine. I'm a member, but I'm also mostly outside the reach of the USPS.