Searching for a device that would provide me with INSTANT hot water without waiting (e.g., water heater), I ended up with the solution of the instantaneous water heater. Having, of course, heard various opinions from acquaintances and reading reviews here and on the internet, and after extensive market research and visits to stores for more specific questions regarding features and consumption, I concluded that the DCE-X from Stiebel Eltron is the best choice in the category of single-phase instantaneous water heaters, as it is the most practical and has the lowest consumption.
The features of the instantaneous water heater simply do not exist, and no other device from those I saw (and there were a lot) has similar ones!
For me, the most important are:
• It has full electronic power control, meaning it is Inverter, which practically means that the consumption starts from about 200 Watts and goes up to the maximum power, i.e., 6 or 8 kW depending on what you choose and what your electrical installation can handle.
• It is the only one that has automatic flow control, the best that technology has to offer today in an instantaneous water heater. Essentially, you request the temperature you want to get at the faucet, and the instantaneous water heater automatically adjusts its power, thus consuming exactly what it needs in energy and nothing more, which means incredible energy and money savings.
• In case the water flow is high and the device's power is not enough to accurately provide the temperature you request, it then adjusts the flow to deliver the desired temperature, and you do not need to constantly adjust your faucet to get the temperature you want, as with other instantaneous water heaters. You simply turn on the hot faucet, and that's it!!!
• Also, in case you are taking a shower and someone else opens another hot faucet, you will not experience a Scottish shower!!!
Secondary features that pleasantly surprised me:
• Easy temperature selection (with half-degree Celsius accuracy) via the illuminated screen.
• Temperature memories and wellness programs, muscle recovery for those who exercise, and protection against colds (!!!).
• Instantaneous or total energy consumption indication, as well as water consumption tracking.
Although it is more expensive than others, it is clearly the most value-for-money product on the market for me!

Let potential buyers pay attention. The device is quite high-quality and aesthetically pleasing. I have the premium model 6/8 kW and for its installation, a thick cable is needed that will come out of the panel. The installation must be done by a licensed electrician and plumber, so you should calculate an additional 100 euros for their fee and the cost of the safety switch in the panel. 90% of homes have a single-phase power supply with a maximum consumption of 8 kW. This means that if the device remains factory-set at 8 kW, you will need to have everything turned off in the house except for a few appliances that consume a few watts in operation. Refrigerator, bathroom light, maybe a television. If you try to use it together with a kitchen dishwasher or air conditioner or iron, then the safety switch in the DEI's turtle will definitely trip. This is because you exceed the maximum kW that the power supply can handle with their simultaneous use. One solution is to adjust the device from the internal jumpers to operate permanently at 6 kW and use it with as few devices turned on as possible. The problem with permanent operation at 6 kW is that it cannot heat the water to a satisfactory degree if the inlet water temperature is low due to the environment. So during the summer months, there may be no problem in the shower, but in winter, in order to heat the frozen water, it automatically reduces the maximum water flow it provides to the shower. The solution is to use a special high-pressure showerhead that saves water in terms of consumption and gives the sensation of higher pressure as it has smaller holes. Another factor that affects the quantity of hot water is the distance of the device from the water supply, as there are thermal losses in a large network of pipes. Personally, I am satisfied with the performance of the 6 kW in the shower as it has no problem reaching the desired temperature of 39 degrees. Moreover, higher temperatures are not recommended by doctors as hot water destroys the natural oils of the skin and creates problems. In conclusion, it is a valuable and high-quality device with multiple safety features that provides hot water 24/7 with low consumption since it only heats as much as you use. For Athens, where temperatures rarely drop below 6-7 degrees, I think 6 kW is sufficient if there are no demands for "boiling" water. However, I will come back with an evaluation in the heart of winter. For now, it is simply a huge convenience to have hot water whenever you decide to take a bath.
Update 11/22/2022: After so many months of use, it remains a reliable and incredibly useful purchase for the home. Although the big cold temperatures have not hit Athens, the water at 6 kW comes out as hot as you want it. The only downside is that it reduces the water pressure quite a bit in order to heat it in time. I have changed 3 different showerheads with small holes that promise higher pressure and water savings from 25 to 50%, but I did not see a significant increase in water pressure. This does not mean that you cannot take a bath, but if you have been used to having hot water with pressure from the traditional water heater for years, it feels strange. In terms of economy, it truly works wonders as the "excessive" 6 kW may sound intimidating, but it only burns for as long as the shower lasts. To better understand, classic water heaters are 4Kw but they burn for 30 to 40 minutes until the water in their tank, which is 60 or 90 liters, heats up. For one person, most of the water goes to waste since it is difficult for them to consume it all by themselves. With the instant water heater, the consumption is 6 or 8 kw depending on the setting, but it only burns for 5 to 10 minutes, which is the duration of a shower. This way, you only use as much electricity as you need for a few minutes, resulting in energy savings. I will come back with a new evaluation in December when the cold weather sets in to see if the 6Kw setting can provide satisfactory water pressure for a shower or if it needs to be adjusted to 8Kw, considering the limitations this entails when using other power-consuming devices mentioned above...
Update 31/1/2023: The temperature in Athens, where the machine is installed, is between 9 and 15 degrees Celsius. I continue to have it set at 6.6 kw with internal limitations from the jumpers. What I have noticed is that whether I set it to 39 degrees Celsius or 45 degrees Celsius, the machine, in this setting, delivers around 38 to 39 degrees Celsius. Therefore, at 6.6 kw, it doesn't have the power to heat the water any higher. Additionally, in order to achieve this temperature, it reduces the flow to 3.2 liters per minute, which is borderline pressure for a shower. Therefore, during the heart of winter, we switch it back to the 8 kw setting, which reaches temperatures above 40 degrees Celsius with a water flow of approximately 4.2 liters per minute, which is satisfactory for a shower. With the 8kw setting, we have the limitations mentioned above regarding the prohibited simultaneous use with other power-consuming devices. So, while cooking with an open kitchen, we cannot take a shower at the same time, and while using an air conditioner, we cannot take a shower simultaneously. We also cannot take a shower simultaneously with kettles, electric irons, washing machines, dryers, or mixers over 2000w. In conclusion, after months of use, this device provides incredible convenience for homes that cannot have a solar water heater. At its full setting, it provides the hot water you need for your shower or dishwashing in the kitchen, but it restricts you from having everything else turned on. This creates an additional stress because if you forget and operate it with another power-consuming device, the circuit breaker trips. And if it's nighttime, good luck... During the relatively warm summer, when the water intake temperature from the network is higher and I had it set at 6.6 kw, I had no problem using it simultaneously with other devices. Also, during the summer, with this setting, you have the desired water temperature at a satisfactory pressure. So, you weigh the pros and cons and decide if such a solution serves you.

The choice of this particular model was made for reasons of performance, water/energy consumption efficiency, and of course safety. Let's not kid ourselves, it's a great freedom to be able to take a quick shower whenever you want without having to wait! Especially for those who have children, they can understand what it means to waste all the water without any savings in consumption. What have I learned about the product and how do I use it up to now? Apart from the economy it offers, the temperature can be adjusted from ~20 to 60 degrees, but it can also be internally limited to an even lower temperature (e.g. 43, 50, 55, etc.) in order to protect children from very high temperatures. This was very interesting for us because, unlike the simpler models on the market, this one consistently provides the desired temperature without having to fiddle with the faucet to achieve the right flow. You simply open the hot water on the faucet, whether a little or all the way, and the water comes out at the temperature you want. Among other things, my husband already found the screen interesting, which shows not only the temperature we have set, but also the energy consumption, liters per minute, time, and more. Finally, I have not yet tried the four different programs for wellness showers, muscle therapy, etc. and I reserve the right to return with a more comprehensive picture. So far, I am very satisfied!