Weeeeell... the principle of these showers is tried and true and applied in many European countries. The point is a) that these countries usually have stringent regulations that govern water quality and thereby whether such a heater can safely be installed or not and b) the specific models of tankless electric shower heads sold in South America can be considered poor designs as they do not allow for the appropriately sealed connection of a double-insulated supply cable or flex. In less fancy terms they all seem to have two loose ends of single-insulated wire sticking out and pretty much the only way of connecting them to the supply is using some kind of flying splice secured to the water pipe. If they had a proper terminal box with glands and someone actually made sure they were only installed in places with sufficiently high-resistance water I don't see anything wrong with them. Reality differs though.
