So after some more reading, I'm now rather considering the Gigabyte B250 instead of the Asus B250 Mining Expert, because shared PCIe lanes are just no good idea.

So what's the catch to be running more than 12 GPUs on the Asus B250? You have to be running GPUs from Asus for the remaining 7 slots, right? If doing it with 12 x GeForce 1070 + 7 x Asus, can it be really running stable?