Sadly, any Goodreads competitor will need to miraculously gain the network effect; everyone you know is on Goodreads, so it'll just be you and whoever you can convince to move to a new platform.
As for the downsides for Goodreads, this blatant lack of moderation is troublesome. I also dislike that Kindle / Amazon are the only visible links to purchase books by default. Amazon already dominates the ebook/audiobook market, so I also simply dislike Goodreads due to their acquisition by Amazon.
Once upon a time I bought something online for about $10 from what was a legitimate business with an address in San Francisco. About 14 months later they claimed I subscribed to some service and started making huge charges to my card ($150/week). Getting them to stop and getting my money back was an enormously stressful and difficult process.
That's why I have a knee-jerk reaction - not a cold, logical reaction - to online purchases from companies I haven't used before, and I'm not the only person like that.
I've had no negative fallout from purchasing a subscription, and they have very good privacy controls for those who don't want the social aspect (so you can basically turn off other user interactions in many ways). It's there when I need it, doesn't seem to spam me and I probably paid with Paypal (I use Paypal as a way to not give 3rd parties my CC info, a proxy if you will) like most things.
It may look old school and appear at a glance like it's not maintained, but it's updated and run actively, there are a bajillion people using LibraryThing. Logging in to look at my account, there's a link on the right for the latest news posted today about "The January ER Batch is up! We've got 2,960 copies of 89 titles this month." (early reviewer books) https://www.librarything.com/er/list