Fediverse

[implementations] are only either partially compatible: pixelfed posts can be viewed and boosted on mastodon but won't display past the first four images

To be precise, this happens for posts between Mastodon and Pleroma too, and it's how improbability or standards simply is. Take W3C for example, some web sites work on Chromium, some on Firefox, and most on both. Or ProtonMail has its own E2EE scheme but still can communicate with other providers. Or a cars and horses both runs on asphalt and dirt roads, but some configurations do better than the others.

not compatible at all: afaict, despite kbin being activitypub based, there is no connection between it and any other activitypub software

Not sure about Kbin, perhaps due to its immaturity, but group are known to work across implementations specialized for it (count Lemmy in as well), and those focus on microblogging, or both, e.g. Frendica.

some of this can be easily chalked up to very different modes of interaction but if that's the case, why advertise as being part of the fediverse when that's only somewhat true? is it just for Buzzword points?

The ability to access to the same infrastructure (i.e. fedi or all interconnected roads) even if less efficiently regardless of what one has is important for a society where people are truly free to choose where to live and who to do business with.

even some sort of shared identity system (hold on while i reinvent openid) would do a lot to mitigate this! but if i want to use pixelfed, i need a pixelfed account in addition to my mastodon account.

Seconded, it would be ideal if implementations are views of the network from the same endpoint. It would require an overspecifying protocol like Matrix instead of underspecifying like ActivityPub or XMPP for a backend to get hold of all necessary data, but that comes with the cost of making optimizations (which require context) incredibly difficult, e.g. in case of PeerTube. IMHO it's not something reasonably achievable, but technology is just part of the equation:

if anything, it's maybe kinda worse because twitter and instagram at least don't pretend that they're compatible, and i don't have to pick a Twitter or an Instagram, there's just one and i know that it'll be the same one my friends are on.

(As an outsider I don't see any functional difference between Twitter and Instagram, but let's say it's Facebook vs Spotify), as stated you won't have to choose between the N friends on X and M friends on Y. Though it might be harder to communicate with those on Y if you're on X, it's better than abandoning them. It's better, yet entirely optional, to be on both.

why the fediverse is better than centralized solutions

More importantly, improbability is future proof. New and presumably doper shits come along and share the same large user base. Old shits can stick around and people their can still connect. No more reliance on network effect and oligopolistic power to force feed ads, mass surveil, manipulate or roll out predatory pricing. Sure, not everyone can make use of the same features, even if that has any significance, socializing ranks higher in the priority list.

many of the problems that exist on centralized platforms (content can disappear at any moment, you are at the whims of the admins, etc etc) exist on the fediverse too, and there aren't a ton of benefits beyond "you can host your own."

Ditto, although it's beneficial to view it in a different angle. Self-hostable isn't just about everyone can just spawn up a server. That's just P2P communication when it comes to social networking. Federation is more than that: though fediblock is a thing, being able to choose your alliance means you can have an admin respecting you and being able to connect with people with different associations, all while minimizing maintenance effort.

Decentralized system also means socially decentralized redundancy, which allows for data preservation even when intentionally removed (by a third party; right to be forgotten is another issue which requires cooperation on data handling;-)

"you can export your follower list and force everyone to follow you on a new account" is not account migration. until there is any story for migrating content, claiming that account migration exists is misguided at best and actively deceitful at worst.

Agreed. Just let's not make perfect be the enemy of good. Not saying there'll every be practical data migration across service providers, given the immense amount of moderation backlog when lack of personal trust from the admin; only wanna point out no centralized service even allow migrating one's network.