Well... The 5.0L getrag is/was built in China. Its often referred to "the china transmission". Its just not very durable for the power and weight of the car. Compounding its issues is the 6-speed auto in the mustang which is AWESOME. They are running high 9's with just a convertor and software. Mid-10's on the stock convertor. Then the china trans breaks running 12.0's and requires a $1,000 worth of shifter mods and stuff to be bearable.
All 3 of my friends that bought 5.0L mustangs refused to buy an auto. All 3 sold there cars after replacing the trans 2 or more times. One guy swapped to the $7k tremec and all kinds off stuff, then sold the car without putting a single pass on it. In the mean time, a friend of a friend showed up and ran 11.20 on a 125shot with an auto trans, drag radials, bolt on mufflers, and a tune. Cats and stock air box still intact.
Sorry... That got long, lol. Yea.... Ford also uses this trans in the new IRS equipped mustangs, saying its "improved", lmao. Aka "we still had too many of them to stop using them".