Afaik the problem is typically not hardware but software, since most server motherboards come with a bmc chip on it. Are you designing a motherboard? All i would ask for a bmc is root access. But now everyone (but the big players, who design their own motherboards) is basically stuck with a stupid embedded linux with shitty software that has half-backed features they don't even need, but now need to care about. When all you'd need would be a way to access a host serial console, read sensors, control boot sequence and perhaps re-flash the bios, ie most likely just basic interactions on serial interfaces.