For me the hardest writing task is rewriting old material. I'm not talking about updating something because the feature changed. I'm talking about reworking and reorganizing material that needs to be updated to fit into a new structure or that needs updating because it just needs to be better.
Writing new content is difficult. The cold stare of a blank space is unnerving. The research that needs to be done before writing even starts can be grueling. The reviews are sometimes long and require rerunning the gauntlet.
The challenge of staring down the blank page and getting to the end of the race is exhilarating. Learning the new information and filling up the page gives me an endorphin rush. The anticipation of the reviews makes the thrill of a good review huge and the challenge of addressing the review comments a savory challenge. The sense of accomplishment makes the pain worth it. Of course it also helps that writing new content does require quite as much groveling during resource planning meetings.
Reworking old content does require a good amount of groveling and doesn't have the same payouts. The endorphins don't rush; they trickle. The sense of accomplishment is hollow because all there is to show for the time spent is a shinier version of the stuff you already had.
Not only is the reward less, The rewriting is also just plain hard. Often times you need to relearn the feature or concept being discussed. You always have to make judgement calls about what information to cut or add. Then there is the angst of reworking another writers (you were another writer when you wrote the original content, time changes every craftsman) handiwork. The cold stare of a page full of words that you need to slay is unnerving and unforgiving.
If you do the rewrite well, then the suffering is worth it. You will have made a significant improvement to the quality of the documentation set. If you it OK, you may or may not have made an improvement. You just spent time spinning your wheels. Often times you will never get any feedback to know finite was well done or not. You will just have to decide on your own.
For me the answer is always that I could have done it better. Nothing is ever perfect and I'm always improving/growing/changing as a writer. I always look at the end result as good enough for now. Maybe that is why I don't write fiction or for magazines or newspapers. In those mediums you finish a piece and are done with it. In technical writing, at least in 21st century Web only technical writing, no piece of content is ever truly finished.