I think a big problem people have had with trying to join federated socials is that they did it when everyone else tried it and thus the system failed. And so people think its really hard when its just been technical issues with onboarding people.

This is a bigger problem than fans of federation acknowledge, though. Human beings have multi-faceted identities, and fediverse instances tend to be quite narrowly focused.

The average person is rightly going to assume that 'A collection of Marxist communities' - to take a real instance description from near the top of the join-lemmy.org list - is probably going to be mostly about Marxism, and not really about, say, knitting, or motorcycle maintenance. And even if a given person is in fact a Marxist, they're not going to see 'a collection of Marxist communities' as a replacement for Reddit, where they can talk about Marxism, motorcycles, and knitting.

You can explain to people all about federation and the ability to interact across different instances, but you're still asking them to make a home somewhere, and that entails choosing from amongst identities in a way that Reddit doesn't require, and that most people don't want to do. You're also putting them at the whim of their home instance in a way that they aren't used to and may not like (what if the Marxist instance defederates / blocks the users from the motorcycle instance, due to their insufficient revolutionary zeal?).

As for being subject to the whims of the instance; that is no different to reddit/twitter admin who also push ideological agendas. In fact, federation allows you to at least migrate to a different regime, something that neither commercial site permits.