Even though I enjoyed module eight's story arc, I didn't actually do the associated campaign on any of my characters, not even my main. Why? Because it was tied to farming heroic encounters which I thought were absolutely horrible.
The problem was that Underdark introduced demonic encounters to existing maps which came in three different flavours, two of which required actual tactics to be completed successfully instead of just blindly zerging the mobs. In one scenario you had to prevent demons that spawned around the edge of the area from running towards the shiny circle in the middle... but people would frequently confuse the fact that the demons disappeared when they hit the circle for a sign of success and actively pull or knock them into it, causing the encounter to fail. The other problematic scenario required you to protect an NPC while they channelled a protective spell, and if they were hit more than once or twice, you were pretty much guaranteed to fail... but you'd try to finish it anyway, just to run out of time with a smidgeon of the progress bar not completed. After repeatedly wasting half an hour or more rotating among events that just kept failing like this I got fed up with it and refused to let my pet tank drag me along to any more of them, boons be damned.
It was only last week that I finally decided to give them another go, and things seem to have improved in so far as more people know what to do now and the NPCs that need protecting appear to have received a significant buff, as I've seen that particular encounter complete with ease even after the Harper got smacked out of his or her trance every other spawn. The circle scenario still fails quite a bit, probably because it's the most sensitive to movement and precision, but people just don't bother with it anymore or even start and fail it intentionally just to get another one to spawn more quickly.