You're seeing it now. Not how it was at launch when we were in much lower ilevel of gear and didn't have red mages. It was the same thing with Keeper of the Lake. In the gear available and the classes available at the time it came out, the final Middy fight was hard in general and very hard on healers. But the thing those dungeons have in common with The Burn is the unforgiving nature of the mechanics. The insta-kill stuff seems to be what SE falls back on when they want to implement "challenge".