I got plenty of Coraline in DT. My green slimer + various other SPS did fine in QT for 45 days. 3 weeks in DT and they're all dead. Just about everything I know how to measure is pretty darn stable.
I'm not concerned about wasting the money I spent because I knew that was a possibility going in. I feel a bit bad that I killed my frags. But what's killing me is that I have no idea why the heck they didn't make it. I feel like one of those mice in an experiment where they deliver shocks completely at random and watch the mouse slowly go crazy.
So my answer to your original question: I suspect there's a large component of just dumb luck involved.
Now I have to go back to work. To make more money. To waste on this hobby. Which is still better than leaving it to my kids.