not if its designed correctly. i don't think you need a run down on how an overflow/sump works but if its designed correctly there are no problems generated by a power failure.
the jury is out on the zflow though from what i have gathered, i have thought about making one and installing it as an alternitive to the regular overflow box, but still have my little issues, mostly cosmetic ones. as long as it removes water from the tank, and doesn't leak, getting it to keep its siphon isn't that hard of a thing to do. the simplest of all options is install an air line and barbed nipple on the heightest point of the overflow and connect the airline to the air - intake of a powerhead.