Fortunately, Gigabyte added the microcode for SKL B1(50653) in latest BIOS(see GB support page). that matches the P-8124(18C) and the P-8136(28C)
The ASRock EPC621D8A(and other) needs a modded BIOS to run P-8124/P-8136.
6 phase instead of 5. the EPC621D8A can not handle 450 Watts. thats not the point. the TDP is still limited to XXX. you can tell it a BUG that the 240W parts do not work on supermicro X11 boards. possibly that is also intended, who knows.
240Watts may be to much over the time, to many RMAs for...
this processors are not needed anymore because the platforms are replaced by 3rd gen scalable or rome/milan.
the price of the 8124M(and other OEM 240W) is so low because there are almost no standard motherboards supporting it.