(Dawnguard spoilers)
1st major bug: Serena refusing to initiate the first conversation with Harkon upon her return.
Solution: Had to reload a previous save 5+ times before it would finally start.
2nd major bug: Moth priest wont finnish his entire speech without bugging out.
Solution: Tonns of times reloading before he would finally finnish it and the quest would continue normally.
3rd major bug: The portal to the soul cairn. You say you are ready and Serena just stands there doing nothing. The portal wont open.
Solution: After an hour of frustration and pain - trying everything, I searched the forum and found a console command that made the quest proceed as normal.
4th major bug: Draw blade to extract bark from the trees. You dont get the bark when using the drawblade and the Q wont update.
Soulution: None yet, and again - tried everything creativity and an imagination allows to circumvent this bug. Currently Im completely stuck at this point.
As it stands, Im tired, frustrated and feel straight out cheated. The hardest boss in the game shouldnt be a recurring bug that happens on every next quest. It feels like the entire quest design was made by a mediocre modder and an amateur.