I think the advantage here is that the average instance size can remain small and relevant to the individual users values without sacrificing the amount of content available in one spot. The owner of lemmy.world (for now) is easier to reach out to and share concerns with. You can't go directly to Spez or to Reddit's future shareholders with a problem in the same way you could with an instance's owner. It's the (im)perfect blend of old school bb forums and the mega platforms.
The biggest issue with the fediverese is the non-transferable nature of your profile. If that can be resolved, I'm all in. Afaik that is a planned feature for Lemmy in particular.