All we have around here are caldaias, which are tankless water heaters. They also control the water flow in the radiators for the house heat. WHen these units are new, they are great, but when they get older, they are a huge pain. Not as simple as changing an element. Not so easy to do a self fix.
We also find the water takes a while to heat. If it is first use of the day, it will take over a minute and of course, winter takes longer than summer. It does take longer for the upstairs to get hot.
I personally don't like these units. They stop working for no reason. Of course, the ones they have over here might be less quality, but this is my experience. In addition, flickering of the electricity is a guaranteed malfunction of the unit. It can take several steps to figure out exactly what it needs to get it back online. Maybe just a restart, or turning the power off at the unit (which is always outside) or shutting the power off completely to the house. Electricity is not the best here, so you can see how this wouold be a pain. it is especially troublesome if you have already started the shower, thought the water was the right temp, and undressed to step in, only to redress and go outside. grrrrr.