A note about terminology: GEC vs. EGC.
IF you're referring to the green wire that's run with circuits, the pipe itself is quite a good conductor. I've witnessed the UL research tests, where the sloppiest EMT assembly you could ever imagine was able to handle enormous amounts of fault current.
HOWEVER, I've also witnessed (and fixed) a 1963 multi-panel commercial service, where the power company main fed a cluster of three large panels, all grouped together. The extremely competent original contractor - the very man who brought the "Ufer" into the NEC - had counted on the conduit stubs connecting the three panels to 'create' one panel. Alas, nearly half a century later age and corrosion had combined to degrade that ground / neutral bond, resulting in massive power quality problems and voltage surges. Simply isolating the ground from the neutral in two of the panels, and running a green wire to the first panel eliminated the problem.
LIKEWISE, I have discovered that in an ungrounded system (480 3-phase, no neutral), using the pipe alone WILL result in all manner of problems if there are ANY single phase (480 volt 2-wire) circuits. Even a failing light bulb will introduce all manner of transients - and finding that fault is extremely difficult.
