I gotta believe it is the rapid extreme temperature changes in combination with vibration and the torque of the engines on the mounts, NOT exhaust pressure that causes failure.
While it is true that all makes of headers crack, not every single header sold by most companies fail. In the time I have cruised this site, I have not seen anyone stand up and say that their JBA's didn't crack. I am sure that there are some out there, but they certainly seem to bee the silent minority.
Kooks has been making headers for many cars, for many years and I am sure they have had their failures, and I expect some of the MN12 Kooks headers will someday fail, I only hope that Kooks success rate and and reputation with the other headers they make carries over into ours.... you are right though, only time will tell..... But you can sure see the difference in the quality of manufacture, that's gotta improve the odds....
'96 T-Bird LX 4.6L Alive & well & living in S. Florida
N/A: Renegade NPI heads/Comp Cams; OEM intake manifold; Bolt-ons
245 rwhp; 13.713 @ 99.35 (Commerce, GA - November 2003)
BLOWN: Renegade NPI heads; Renegade shortblock; Intercooled T-Trim; some other stuff
Pump Gas Numbers - 547 rwhp / 525 ft.lbs; Best Time - 11.388 @ 118.68 (Commerce, GA - November 2007)