Here's my personal experience with these platforms:
I'll start with duh-duh-duh-duh! Tapas!: From my experience, this is a fantastic site to get to know other authors and build bridges. Most people are super polite on the forums, and doing read-for-reads on this site has been pretty nifty. I appreciate that there is no rating system on this site, either, so a person has to actually read a work to gauge whether they like it. I also appreciate that Tapas occasionally features smaller creators--that's how I got a decent following on this site. The only real drawback I can think of is that you really have to commit to the romance umbrella if you want a consistent following, and that novels really get stuck in the shadow of comics. Additionally, Tapas just doesn't feature smaller creators to the same degree they used to.
Next, I will go over my experience with Wattpad: Holy smokes, for such a popular website, I have never felt more invisible as an author than I do on this site right here. No views, no comments, no likes, no nothing! Again, this site seems to favor romance... and not the kind of romance I like. XD I'm spacing right now on whether the site does ratings--I don't think they do?--which is kinda nice, I guess. I do want to say however: You should probably at least TRY putting your story up on here, because if it gets featured, well... you've got an audience.
Next I will go over my early experience with Fictionate.me: Honestly, I like this site a lot so far. I think the readers there actually appreciate the most underappreciated genre of all: sci-fi, and I also appreciate that you can put your story up on there for some mon if you so choose. Haven't been on there long at all, but I got some engagement, and I also really like that your story automatically gets like, an audiobook version when you throw it up. XD It's like a robot voice reading it, but it's still relaxing and good if you haven't got time on your hands to actually read. The drawback is that the interface is a little bit hard to get a handle on. Also, I don't think this site does ratings either, which is nice. The front page of the site looks a little messy, but I do like the fact that they lots of different rows like: Consistent writers (my favorite), new books, trending, and top paid books.
Next I will go over my early experience with Neonovel: I really, really like this site so far. I think the overall design is really appealing and you get a share of the revenue from your story. Additionally, I think the interface is really easy to get a handle on, and I really like that the site has a specific row on the front page for new stories, popular stories, fresh trends of the month, etc. it feels like they give everyone a chance in the sun, which is awesome. The site does do ratings (which I don't like) but it also does reviews! (which I do like!). I encourage everyone to give it a shot! My story was also featured as a fresh trend and a sensational arrival, which I really appreciate. <3
Moonquill: This is a pretty good site, too! I got on the featured novels list, and I have gotten quite a few views on the site. There's not really much engagement, but at least I know people are looking at the darned thing, you know. XD I think the design of the site and the interface is pretty self explanatory, and I love, love, love, how you can separate your book into volumes on this site! <3 The site unfortunately does ratings, but it also does reviews, so it balances out. They also have a spot on the front page for stories that have just updated! This site has a variety of different popular genres, which is really nice.
RoyalRoad: To be honest, this might have been my favorite site of the bunch for these reasons--it has an awesome interface, it features authors who have just updated right on the front page, and I got a lot of engagement even without doing review exchanges. The interface and design of the site is also gorgeous. However, I found the readers to be, like... incredibly picky and kind of rude? XD I also hate the ratings and reviews system. Like, someone can just come by and leave a bad rating on your story, but not a review, and you'll never know what bothered them about it. And people do this A LOT on this site. A variety of stories are also popular on this site (even though it does lean shivers Isekai). I do plan to continue uploading there again eventually, but man, I needed a break. XD
Last but not least: Smashwords: Not a bad site at all! You can price your book however you like, and you get part of the cut. Engagement is okay, and I've actually had a few sales on the site (even though most of the stuff is up there for free). The biggest pain about this site is that it's a pain to get into the premium catalog. XD Romance seems popular here, too, which kinda sucks for me.
And that is my longwinded review of each of them. I hope you enjoy. XD