My ASUS Z97 Deluxe doesn't like my Mushkin RAM @ 2666MHz. I had to lower it to 2400MHz to get it performing properly. It runs @ 2666MHz but not very well. The same RAM ran brilliantly in my Z77 ASRock OC Formula.
i spent an hour trying to work out why all monitoring software (CPU-Z, Aida64, HWInfo etc) were reading the BCLK as 99.8MHz. I ended up leaving it set at 100MHz in the BIOS but changing it in XTU to 100.2 (a bit over but as close as i could get it). Now it's reading 100MHz again.