I feel your pain. I've run the Prison as much as I could. Yes, on the lower levels (Waiting for my regular fireteam to do the higher levels where etheric light is guaranteed). I ran the nightfall on all 3 chars. Still nothing. Hopefully Iron Banner will be back soon. Guaranteed etheric light at levels 3 and 5. So there ARE guaranteed ways to get it, but we really have to work for it. I think its Bungies response to the "you didn't EARN IT" argument. Now you really have to work to upgrade and get powerful. I'm cool with that. But yeah, it sure is frustrating.