With great pain, I am writing this review.
The mobile phone is indeed flawless in all aspects - a tool.
I took care of it and it looks brand new.
Until a few days ago, after 3 years, it started constantly rebooting without any reason and without even turning on.
I sent it to the company and, of course, I was disappointed by the fact that they want 207€ to change the motherboard.
When you have spent around 700€ on a purchase, you expect it to last for 5 years.
Furthermore, I had an even worse experience with the company and their after-sales service, as they asked me to send the device at my own expense (ok), but they also charged me for the technical inspection without informing me, while I could have gone to another service store that repairs motherboards instead of just replacing them and in a shorter period of time (it took 10 days to receive a response).
So, I give it 2 stars for the above reasons, because the flagship of the company should have been more reliable and should have anticipated beyond the warranty.
(This concerns the 6GB/128GB version)

I got the phone in July 2019 and I still have it. The best phone that has passed through my hands. The battery still holds incredibly well, it has fallen 100 times without a case, it has taken a regular dive in water and it was as if the phone wasn't touched. I feel sad when the time comes for it to break. Which will definitely happen at some point.

After 1.5 years of professional use (not gaming), my opinion is as follows:

BATTERY: Lasts with reasonable use for 1 - 1.5 days (if you don't constantly have GPS and data on). Charges quickly (full charge in about 30-40 minutes).

SCREEN: Decent with large brightness range.

NAVIGATION: Very satisfactory speed. It's not an iPhone 12, but it doesn't disappoint.

CAMERAS:
A) Photos: Did not satisfy me. The format that produces correct quality is 4:3. In wide-angle focus, another camera is activated which significantly reduces the resolution of the photo. Night shots are not good and let me explain: I also had the Huawei P9 and the photos using the "night" mode were stable and clear. On the other hand, with the MATE 20, even though the device is perfectly stabilized, the photos have ghosts, as if they have slightly moved (e.g. in landscape photos, the phenomenon is intense in light sources, i.e. street lamps in an urban landscape or stars in a natural landscape and complete darkness)! Without any critical intention, the result in this case is simply unacceptable.

B) Videos: Satisfactory video and sound, with slight stuttering during device movement. The major drawback here is the adaptation of the device's brightness according to the focal point, resulting in one area being too bright and the other area being excessively dark in the video.

MICROPHONE, SPEAKER, AND EARPIECE: Clear audio and speech. The speaker is loud.

FINGERPRINT SENSOR: Fast and accurate in response.

GAMING: I have tried games, but it's not meant for heavy gaming. The resolution for slightly more demanding games than average will be at best medium, while for heavier games, you will simply curse.

RAM: 128 GB is a good capacity for the device's data. The extra slot for a memory card is practically USELESS because it can only accommodate HUAWEI cards, which are probably prohibitively expensive to acquire.

EXTRA FEATURES: The infrared is useful for using it as a remote control and the audio jack for headphones.

DURABILITY: Unfortunately, the back of the device is covered with glass, which does NOT tolerate falls and impacts. The genuine replacement costs €100 with the labor for replacement (wtf...)!

Overall, it is a phone above the baseline, which, however, cannot hide its age and distance from today, especially in terms of the cameras! Therefore, if your main interest is focused on the cameras, I recommend bypassing it.

In any case, with a purchase price slightly below €300, it was a decent choice.

HOWEVER, under NO circumstances is it worth €400, which is its current price, given that with a little more money, you can go for more comprehensive options (even if they are not Huawei, which, due to the lack of Google apps, has become a company to avoid), while with a "small" increase, you can even get an iPhone 11, which truly offers a complete package of services, speed, photographic quality, and reliability.

Note: This model is NOT imported to Greece by the official distributor.