Can I mix nvidia and Nvidia Card on the Asus z270a easily?
May you mean AMD and NVIDIA.
Because of the space between the three socket PCIe 16x on that board far enough, you can add the three card directly to board without using Risers.
Can I add 3 1070 GPUs directly on the board without issues?
Yes, you can mix both AMD and NVIDIA on one board, for better mining process, use two different mining software to handle each type of cards. But not too easy to do it.
Advantage of putting the cards directly on the motherboard.-You would be cutting some cost to buy a Risers.
-You will save PSU cable socket (SATA, MOLEX, 6PINS) for powering the Risers.
Disadvantages of putting the cards directly on the motherboard.-Those cards will get in the way other PCIe (PCIe 1X) sockets near the card that plugged.
-Will heat faster, because of cards position close to other components on the mobo that the same produce a heat.
