I tried an ASUS H170 Gaming board and only 3/6 slots worked.
4/6 was possible depending on the right slot assignment and UEFI Setup configuration but it wasn't really stable under mining load for longer periods.
5/6 was a disaster: the misbehaving SATA controller caused data corruption on the root filesystem up to the point where Windows became unbootable.
6/6 was better, it didn't even get through the firmware initialization, so it had no chance of corrupting data on the SATA drive.

I took it back to the store and left with a 6-slot Gigabyte Z97 Gaming 3 (v1.1) because they had one of those at stock. It works great so far (and it looks like a decent PC mainboard unlike those 80/90-series 6-slot Asrock boards, so it might be easier to sell when the time comes...).