I recently replaced the clutch and installed a Steeda firewall adjuster & FMS quadrant. The stock cable wasn't long enough to give me the free pedal travel I wanted, so I replaced the stock cable with an FMS adjustable cable. There was no difference in the amount of effort needed to depress the clutch. I had used a FMS adjustable cable on my previous Mustang for more than 5 years and had no complaints.
really, the problem with adjustable cables is not the construction so much, as it is the extra length. the adjustable cables are a good couple inches longer or so. This extra length makes it impossible to route with creating a sharper bend. This sharper bend created from the extra length causes more cable effort, binding and snapping.
I run an adjustable cable, but on my 66 mustang, and it's routed perfectly and i've had 0 issues with the adjustable cables.
as well, some people spend a buttload of cash on the oem cables. I've used the $20 autozone stock non adjustable cable more than once. It's always held up, AND has a lifetime warranty