1. Install Qt Creator test client via online installer with Boot2Qt 5.7 embedded package including any number of device images.
2. Launch Creator -> Tools -> Flash Boot to Qt device -> <any device image installed in step 1.>

Result, two things may happen:
1. Error note stating writing error on sd card, without even starting to write the image (I've seen this only a few times) -> see attachment 1
2. image is seemingly written to sd card, progress bar goes from 0 to 100 and by the looks of it, everything went fine.

Looking at the sd card content, flashing process outcome may be:
1. Looking via file browser the content looks fine. Using the sd card in actual device is successful.
2. Looking via file browser the content looks fine. However, in the actual device the sd card does not work.
3. Looking via file browser the content looks scrambled and the sd card is unusable -> see attachment 2.

There doesn't seem to be any pattern how any of the previous options happen. Re-flashing image enough times seems to result in a correct sd card image.