I DFed Thornmarch extreme with no problem.

The PROBLEM here is something even the most hard core static groups mentioned.

The boss battles have FAR too many insta kill mechanics and super damage (normally insta kill) hits. Instead of a fight, it becomes a sort of memory reaction game, that is a bit more stressful than fun. If anyone of your team dies, it generally kills the whole team. This is why DF fails and only preformed groups have any real chance. A bunch of random people and new players won't be in sync to take on these heavy hitters. Once a healer or tank dies, it's game over most of the time. This style of bosses is incredibly troubling and problematic for the style of gameplay.

So the problem is not with how DF is, it's with how boss fights are structured. The player base merely adjusted to this issue by relying on preformed parties for a much higher % of serious attempts instead.