1) the brass plate was strongly curved and shaped like a wave. I had to work with tools and level it.
2) VIA/KMK initially did not have a configuration file. Upon request from support, the file was issued. However, it was issued completely broken and not valid. I had to study the format, files of other keyboards and manually correct the issued file. Only after this it became possible to use VIA.
3) I discovered that either the KMK firmware or bugs prevent the use of all the keys, for example, on a Mac, mission control and laychpad (keycode KC_MISSION_CONTROL/KC_LAUNCHPAD) is not recognized.
In general - nice, thocky keyboard, but not buy-n-use solution.