One thing that I have often wondered about is how effective are these devices for the elimination of flying insects. After all the light attracts insects to kill them. So might it not produce a cloud of pests just hanging around that have not yet been killed? Though the satisfaction from the electrocution might be compensation for this in some people's minds.
I grew up in the Black Forest (not the famous one). Out near the back deck we had a bug zapper hanging from the limb of a pine tree. Hundreds of the local moths would get there, perhaps thousands. Every year the Miller Moths invade, getting into every crack they can. Hiding along the route of their migration. And thus the primary reason for the electric bug killer.
One day I was fascinated to see a blue jay perching on the side of the bug zapper. It was poking it beak into the interior space. I was sure that it was going to hurt itself, but it didn't, even when it pulled moths right off the electrified grid. Clever bird. We rarely had to clean the thing out because of it and its successors.