They can cause damage even if installed correctly, if their design is deficient. That's the problem with them.
The crank pulley has a damper on it, a ring of iron adhered to the body with a stiff elastomer. That assembly is specifically tuned -- mass, diameter, durometer of the elastomer etc -- by Ford to dampen torsional vibrations of the crankshaft that show up at the snout of the crankshaft.
There are eight power pulses being fed into the crank every 720 degrees of revolution and because of the length of the crank, it's going to bend ever so slightly with each cylinder's power stroke. When you have eight cylinders feeding in power pulses continuously, there may be points in the rev band when the power pulses are being fed into the crank that match the natural frequency of the crankshaft (or one of its harmonics). When this happens, you can get constructive interference and a rapid increase in the amplitude of the overall vibration. Think of the Tacoma Narrows bridge to see the effects of constructive interference.
The damper is there to alter the vibration characteristics of the system by acting to dampen them. When you change the pulley to a really small diameter damper with little mass, you've taken away the only mechanism to dampen crank vibrations which Ford went to considerable engineering effort to design.
Because the oil pump is driven directly off the crankshaft (right behind the damper, timing wheel and crank sprockets), and (in part) because the OEM oil pump gears are powdered metal, it is possible for these vibrations to pound the pump gears until they shatter, usually taking the entire engine with them.
Piggybacks get a bad rep but at least they preserve the factory damper mass and design...