Substack is newsletter focused, subscriptions are for individual substack writers’ newsletters (you can’t access all substack newsletters with a single subscription) and it has a recommendation feature that writers like because it can help them grow their subscribers and therefore grow their revenue.

I’m not a fan of Kagi’s founder, so I generally don’t use it.

Mojeek

Lemmy.world has no lock in on their “power”. They have the most volunteer labor, money, and infrastructure. That’s makes them stable, so people aren’t worried about their data suddenly going offline (like kbin) and they don’t worry about the service being flaky.