Natural Gas/Propane water heaters

Looking for opinions regarding electric point-of-use water heaters such as the one pictured below. My main concern is, are they an electrical hazard if installed properly. (Sorry for the spanish. The english side of the box is covered by labels.) [img]https://cimg2.ibsrv.net/gimg/www.doityourselft.com-vbulletin/2000x1504/img_3874_c9dde8b99a4682cfda6f52b760171f55eab9e6e6.jpg[/img] [img]https://cimg5.ibsrv.net/gimg/www.doityourselft.com-vbulletin/2000x1504/img_3875_9adbd50b5138154bbb2ed77957ef4fcfc2326173.jpg[/img]

I am having issues with the operation of the thermostats on a 4 year old Rheem electric water heater. We began running out of hot water in the shower and upon testing both thermostats and heating elements we determined that the upper thermostat was faulty and only the lower thermostat/element were functioning. Rheem sent both new thermostats and lower element because after testing both elements the Rheem tech felt the lower element was not within specs. The power supply to the heater is 240-243 volts and the 5,500-watt upper element tested at 10.3 ohms and lower element at only 9.4 ohms. After replacing both elements with Rheem SP10552PH resistored elements, and both thermostats with Rheem supplied thermostats, when heating is demanded the upper thermostats automatically sends power to only the lower thermostat/element unless the upper thermostat is set at almost 150 degrees. Even when the lower thermostat is set to 90 degrees and the upper thermostat is set to 125 degrees the unit sends power to the lower thermostat/element. When I was replacing the thermostats and elements I replaced the annode and inspected the dip tube. We have grandchild visit often and are concerned with scalding with water temps at 140 degrees. [img]https://cimg3.ibsrv.net/gimg/www.doityourselft.com-vbulletin/2000x1504/rheem_thermostat_f0cd8ef2362068766a4991bd57eb7984a37f098c.jpg[/img] Am I missing something?