The drop rates for skeleton keys are the same as the drip rates were for strike loot pretty RoI (I think). It's likely that all this whining about key drop rates is due to the fact that peeps can see the chest at the end and it bugs them that they can't open it lol. I farmed psion flayers nightfall many times before RoI and still don't have any of the flayer mantles. Drops rates are fine cuz theiyr're supposed to be very rare.