They work but the values are meaningless, as they are calibrated for wood. They measure conductivity and the minerals etc in plaster etc are very different to wood, so are not reliable for measuring water content in anything other than wood...
There's a great Youtube video somewhere showing how you can use one of those meters to con a homeowner into believing almost any wall has dangerous levels of damp.
They work but the values are meaningless, as they are calibrated for wood. They measure conductivity and the minerals etc in plaster etc are very different to wood, so are not reliable for measuring water content in anything other than wood...