So, Substack is nothing like Twitter. I personally like it better, but it's a fancy mailing list as far as I know - provides subscription options and a web archive. I honestly think this microblogging is not really want a scientist (or anyone who has nuance in their writing or opinions or thoughts) would want - it's a soundbite at best. So going back to basically a mailing list (that operates like a blog) is a big improvement to me. And providing monetization for the posters and platform via subscriptions seems more sustainable for more types of content.
I can see some people who thought Twitter pre Musk was golden going for Bluesky simply because of the founder's personal brand. Otherwise, I have trouble thinking why people would go for Meta's anything new - given how poorly their new products have gone recently (metaverse?). So in the free and open, of course Mastodon makes sense, and I'm on that, but I don't find any community or real discussions on it. The format just isn't conducive to that IMHO.