I have two KGPE-D16's running coreboot 4.11. One runs as my server and uses FreeBSD, the other is my desktop and dual boots FreeBSD and Linux. Both work great. Both have 64 GB RAM (4x Kingston KVR16R11D4/16). You need to remember that the manual part for positioning RAM sticks in the ASUS manual is wrong, I needed to put those sticks into other slots (this was also true for ASUS BIOS).
One other issue is that redirecting SeaBIOS output to serial console makes video output unavailable if you use AST2050(you get only serial console). But once OS is booted, it takes control of that, so you can have both serial and video after booting. Using both consoles works in SeaBIOS when using external GPU, however.
Other than those two issues, boards work great and I'm kind of curious why people have so many problems.