As someone who is just a reader the main annoyance at FFN is that when a story gets a new chapter it is often hard to get it to actually show the chapter. Sometimes you can only get it if you try to go directly to it, sometime you can only get it if you go to it from the previous chapter. Sometimes you can't get it at all.
Also sometimes if you try to reread the next-to-last chapter to remind yourself of context before reading the new chapter it say the next-to-last chapter was not found. As with the last chapter sometimes you can get it if you approach from the right direction.
Sometimes it is even the last several chapters that get temporarily lost this way.
So far this eventually has cleared up on a given story but sometimes it takes weeks.
From what I've seen authors mention there are also several issues with its support for authors making them as frustrated and annoyed as us pure readers.
Recently I've also noticed even the browsing story categories is falling apart. E.g., go there and from the "Browse" menu pick books. On the books page pick "Harry Potter". Then when it lists the HP stories click the "Crossovers" link to see the HP crossovers. That gives an error, and has done so for quite a while.