Xiaomi has a deserved reputation for low quality. Generating heat via electrical resistance is the most costly and least efficient method. Natural gas or heat pumps are far less expensive and environmentally friendly to operate.
42 litters per what unit of time? What the heck does it mean 42L continuously? Maybe 42L per minute?
Not everyone has gas pipes in his flat around. Electricity is more common. I was looking for some similar device for my bathroom where you do not need constant warm water.
On Europe now gas is much more expensive than electricity.
For heatpump you would need an additional part outside of the building connected to the heater. It is very difficult and with a lot of hassle to install it after a building has been built.