We assume the factory did its due diligence as well, and yet there are many cases (TSB's, recalls) where we know the factory fell short too. And that was before all those parts were 50 years old, and we consider that nothing made 50 years ago is as strong as it was the day it was made. Heck some of the stuff that's sat unused on a shelf for the last 50 years isn't as strong as when it was produced. Be educated and informed, but at some point you just have to hit the street.
The upper control arms don't support the weight of the car like the LCA's do, the load requirements for them are quite a bit lower. Yes, their failure could still be catastrophic, but the margin for error as far as strength goes is MUCH different than even the LCA's.
Sorry, but if we're going to be technical even if they have a fatigue strength that's only 10% of the parent material that's only a single order of magnitude, not "orders of magnitude".
And I agree, if the strength is truly only 10% of the parent material that's not good. But that would be a pretty extreme case, and I think the fact that the UCA's in question have been used on the street for at least 2 years would demonstrate a fatigue strength that's substantially better than 10%.
"Orders of magnitude" would mean that the welds would have to be hundreds of times weaker, at minimum. For example, Rick Ehrenberg tested factory LCA's in failure in the April 2011 Mopar Action. He did a two point test (supporting the LCA at the ball joint and pivot) and used a press to add pressure until the LCA's failed. The stock LCA failed at 2,800 psi. Now, an order of magnitude would mean a failure at only hundreds of psi. And "orders of magnitude" would require a failure in the tens of psi. The factory UCA's wouldn't take as much force before they bent, the LCA's are far stronger. But even if you assumed the fatigue strength of the UCA's is in the thousands of pounds, that would mean the fatigue strength would have to be in the tens of pounds to be literally "orders of magnitude" lower. That wouldn't hold up on the street for any length of time.
So yes, that's being pretty technical. But so is using the term "orders of magnitude" correctly, and it is a technical term.