You're not going to find many hard facts about the failures because there aren't that many.
What you'll find about the successes are that many do in fact "overload" the ratings of vehicles daily and can only offer their opinion because there isn't an engineer to stamp the new "rating" to make it a fact.
Youll have to form your own opinion about what you consider to be a "safe" load for your setup.
I grew up kind of like Lambo watching and participating in work activities that would just bury vehicles. In high school I turned wrenches for a landscape nursery and thanks to that experience I learned how to weld frames and replace springs and axles! I saw what the trucks (weaker than new ones today) endured day in day out and I saw what/when they failed.
When a brand new 1 ton 1989 Dually gasser is grossing 40klbs gcvw and it takes the upper limit of that to actually start breaking things, I don't feel too bad with 4000lbs in the bed of my truck occasionally, on OE rims.