It's basically easy and cheap (once customer base grows from spooks & diplomats to general public) but it will seriously hamper your 4G, and wifi too if you want intra-building insulation.

It's called metal.

Can you elaborate?

Faraday's Cage

This is covered in the Range-R FAQ:

http://www.range-r.com/FAQ/index.htm

The RF bands they're using apparently don't go through metal so well.

By reading the article I would have thought it would be possible to walk down the street while pointing at a house and the device would show if/where someone is hiding but after reading this FAQ I don't think this device is working so well as people expect.

Maybe in houses with wooden walls but not in buildings with thick brick or concrete steal enhanced walls. They even recommend to press the device against the wall/floor to get the best results. This means that the device can be used to see if somebody is behind a specific wall but not just somewhere in the house a couple of rooms away.

In the warmer parts of the country, you are not looking at a lot of metal/brick. In California for example, you are looking at a lot of plywood and insulation mostly.