Those scales work by sending a small electrical current through the body. It then measures how long it takes the current to travel, in the case of the scale, from one foot to the other. Fat is an electrical insulator, so theoretically if you have more fat the current should travel slower. The problem lies in the fact that many factors affect electrical impedance - hydration, dryness of the skin, the way that fat is distributed in the body and what part of the foot the current is going through.

So yes, they are extremely unreliable, even the expensive ones. If you want to make the reading as reliable as possible make sure your feet are slightly damp and that you have no lotion on them, find a way to make sure you are always standing in the same location on the scale (be as exact as possible), and try to ensure that your hydration is the same. The easiest way to do that is to follow a hydration routine and weigh yourself at the same time of the day - morning is better.

It still won't give you a true body fat percentage but you'll be able to use it as a tool to tell if you're heading in the right direction as mentioned by LBTC.

Personally, I don't bother with them. Too inaccurate