Don't get caught up with what they are saying as gospel. Theory does not apply to reality sometimes.
And sometimes it does.
Snap oversteer due to rear roll bind does exist. Ask Jack Hidley. he has published a study done by Maximum Motorsports examining rear roll bind in several common rear suspension configurations. This isn't something that just exists in theory.
I am still on the original UCA's and they still look good to me after over 8 years with a panhard bar.
Do you road race the car with sticky race slicks or R-compound radials? Open track events? HPDE weekends?
If not, you may never have loaded up the rear end enough for the suspension to bind.
And even if you have, the increased roll resistance because of bind may not have been enough to make much of a difference in the rear grip. Depending on that bushings you have, what tires you are using, and your driving style, it may net be noticeable even then.
How replacement UCAs are installed, and suspension setup and application can also play a roll.
Err, running a Panhard Bar is part of that suspension setup and application.
I can almost guarantee you, if a Mustang is being driven hard, especially with sticky tires and upgraded LCA bushings, that the UCA busings are going to eventually get torn up. In my case the chassis bushings were fine, but the diff bushings fell out as soon as the upper arms were removed.
No, I'm not claiming you're not seeing snap oversteer, but one incidence does not prove the point. Now if you can point to dozens of cases of track-driven Mustangs running two UCAs with a PHB and no snap oversteer, that'd be different. I CAN point to dozens of cases where they DID have snap oversteer.
One reason something can be common knowledge is because its true.