Main issue with the Z170 chipset is that it is almost totally reliant on the CPU to provide PCI-E lanes - some older chipsets + optimal CPU could provide up to 40 lanes, Z170 seems to be limited to about 28 at MOST with any CPU it works with.

 They're just not intended to run more than about 4 video cards, and have to some convoluted stuff to be able to do so.

 Onboard sound and LAN do not normally use a PCI-E lane.
 Some M.2 SSDs do use a PCI-E lane.