Writer's perspective - starting and ending a book are the top 2 hardest things, in my opinion, especially because editors get all up your ass about beginnings. I suspect this has to do with primacy principle, i.e.the editor is "most aware" during the beginning of the reading, and also aware that readers do the same thing. They will pick up the book in the bookstore and possibly skim the beginning to see if they should buy, so you gotta hook.

Other things I find difficult about writing:

Ending chapters (I saw a really bad one once in the Broken Earth trilogy that was basically "[character] realized she had nothing more to say, so she walked away." I totally understood how the author got herself in that situation lol)

Making decisions on tropes. For example, say I'm writing a sci fi with a "rise of the machines" plot point (I am). What is the nature of those machines? Rampant AI in existent technology? Did it make war-bots? Or maybe multiple rampant AIs in autonomous bots? Etc. There's like fifty of these kinds of decisions I need to make in a book I'm working on now.

Writing dialogue that's interesting, but not campy, but realistic, but not drab.