What causes failure in longer USB cables?
The 28AWG wire has resistance of 65 mOhms/ft, see Wikipedia. With 7 feet two ways (ground and VBUS, = 14 feet) the total parasitic resistance added by the wire is about 1 Ohm, if account for connector contact resistance. 

An external hard drive can take 800-900 mA of current, some up to 1.8 A when spinning up, which would degrade the input voltage by about 1 V (1A over 1 Ohm). This drop would be out of specs and is usually devastating for mechanical hard drives.

So yes, in this particular case the weak spot is power delivery over 28AWG wires. The next hurdle would be the impedance (mis)matching across the cable data wires and soldered connectors.