Lemmy is one such alternative. I follow a few instances.
* Lemmy has much more "extreme" roots. The original people who went to Lemmy back in 2018/2019 have very strong political ideologies, and even though Lemmy is much bigger now, those roots really shaped the culture of Lemmy
* Lemmy is federated, sort of like email. You can self-host your own email provider if you really want, but you should probably just register to one of the top 3 providers
* Lemmy is missing niche communities. At around ~1% the size of Reddit, you just won't find a community for your favorite game, city, or meme genre
* Lemmy is obviously open source (AGPL), and their servers are very cheap to run (compared to Reddit's insane op costs)
* As a poster, I enjoy "contributing" quality content to Lemmy. No one is getting rich or IPOing off my work, and it is clearly designed in a way that even if the founders wanted to make it profitable, they couldn't
Tl;dr For KPIs, Lemmy is objectively worse than Reddit. The only real reason to go is if you like the "alternative" social media services.
And because of that, many instances also block many other instances, leading to fragmentation and people seeing (or not) different sets of comments... it's very much a "which wind would you like to piss into" situation IMO.
I always feel like my comments do not reach many people and am questioning why I even bother to say something.
I would say the exception to this is communities reddit frowns upon. For example, piracy related.
It's still in invite-only mode, though.