Hmmm... I have them on Asrock 970 performance Mobo with an additional connection to power the PCI's.
Additional power is the key here.
They have been running around 60 watts each for several months with no issue. I also have 2x 1060's on risers (connected to the PSU) with no issues from the mother board. Its actually one of my most stable platforms because I don't really tweak the settings. I am not sure what the 750ti's were pulling through the Mobo but I guess I will have to keep an eye on it.
295x2 (500w card) 10 watts through mobo, 750ti (35w card) 35 watts through mobo (no pcie power connector model). So you never know.