Well that's not entirely true.
It does share the 8.8 axle, and the 2v 4.6, but there are differences, non-interchangeable differences. But really when you get down to it you should say the f150 has a mustang drivetrain because for "truck" duties that setup is cutting it very thin. Some of it's just not stout enough for heavy duty work.
The 99-04 mustang's aren't bad though, they're just garbage from the factory. You need to cut down on pointless weight, get a panhard bar, upgrade the arms/bars on the 4 link, coilovers, and run a good 2-4" stagger in the back to make the rear end more stable and predictable. Sadly it's a widely untapped market. Most people won't go to those lengths because everyone just assumes those cars can't be made any better.
Part of it has to do with modular engines being pricey to build as well though, but they love to be revved and run hard so I can't imagine them being a bad engine on the track.