I bought the item you talk about 2 in fact and to be honest I never really noticed a difference re spiders in the house, but I wouldn't say 100% no because I bought it fairly early on moving into a house so it could in theory have been worse without the device. Problem I had with the device is they do sometime effect small pests like mice and rats, problem with that is if you have small pets in the house like hamsters or guinea pigs it would drive them into an early grave.
The item basically sends out a noise that humans and large animals cannot hear but to a small insect it is a really annoy sound like a human tying to live in a house with a smoke alarm going off 24hrs you couldnt stand it and you would move house wouldnt you, so the spiders move outside or dont come in in the first place.
Mine were wall mounted, looked like a smoke alarm you can get ones that plug in and send signals all round the house through the house wires.
I have bought 'cheap' insect electric shock lights that are supposed to attract and kill flies but they never attract??
Best thing for flies are fly papers in the form of rolls (best) and sheets you hang up they look terrible but at the end on the day if the flys come inside that much and annoy you it might be right for you.
I have made my own fly screens with tenson rods and fine weaved nets, i either have one tension rod at the top and weight the bottom of the net to keep it from blowing open (good for doorways) or put tensoin rod top and bottom and sllde the net over each.