On an event I deleveled -32 levels with no fails at all. Other days one fail in 5. Yet other days 1 fail in 10.
Some days you are lucky, some days you are not.

I would recommend that if you are going to de-level somethin expensive, do it during an event. I've seen folks de-level Sedorians down to 1 during event.
It is the best time to get the most bank for the crone.