The circuitry was not nearly as effective as a true cloaking device: ships employing them being somewhat detectable (as a "reflection") when they moved into certain angles from the sun. Ionization trails might also be detected. Furthermore, the effects of the circuitry could be temporarily defeated with a polaron burst.
Tuvok later speculated that the ship had been able to deceive the ship's sensors because someone aboard Voyager had sent instructions on how to perform such a deceit. (VOY: "State of Flux")