This site is not as commonly used nowadays, though it was more popular five or ten years ago, so if you're looking for works for a fandom that hasn't been active for a while, it's possible you'll find more on here.
Wattpad definitely has a reputation for fanfictions that aren't as high quality, but it has some features that the other two sites don't, like the possibilities for cover images.
Archive of Our Own is definitely the most popular fanfiction website out there. It has the most sophisticated tagging system and is where most people post their works nowadays.
Tumblr is still a very popular spot for many fandoms, and is definitely not as dead as the rest of the internet believes. It has a very specific culture that is not for everyone, but you'll find more fanart, debates, and critiques (good and bad) here than on a fanfiction website.
Twitter threads can lends themselves well to brainstorming or outlining, and can sometimes serve as a way to write a mini-fic in a unique format. Be forewarned, this is the site that tends to have the very argumentative and aggressive sides of fandoms.
Fanfiction, like any genre of fiction, has things is does better, and things it can't do as well. You shouldn't go to a romance novel if you want intrigue and mystery, or a science-fiction novel for rom-coms, after all. Because of the way fanfiction is written, with focus on pre-existing medias, there are some things it does really well, and others it doesn't or can't do as well. I've listed below some of the things I've noticed throughout my years in fandom.
Strengths:
Weaknesses:
1. Writers are doing this for free, in their free time. They are under no obligation to give you anything, for most of them, this is a hobby. Comments demanding updates or telling them that they need to write faster are in poor taste. Even if you mean it as a way to show you're enjoying the story and are excited to read more of it, simply saying that would work better. "Please update" comments don't feel as good, and can actually kill motivation to work on something.
2. When it comes to fanfiction, toss the "don't interact with old posts" rule out the window. It is not creepy to comment on an old fanfiction, or to comment on every chapter of a multi-chapter fanfiction, for that matter. In fact, if an author gets a comment on an old fanfiction, it often makes their day! If it's incomplete, sometimes it can even inspire them to start working on it again (though the "don't comment please update" rule would still apply here. In fact, it applies here the most). Don't worry about when and why a fic was written, if you like it, say so! It will be very much appreciated.