I read this recently on Nostr.how and it summed it up nicely.

Mastodon (and similar)

User identities are attached to domain names which are controlled by third-parties.

These third-parties can ban you, just like centralized social media platforms. Server owners can also block other servers.

Migration between servers is difficult and can only be accomplished if servers cooperate.

There are no clear incentives to run servers, therefore they tend to be run by enthusiasts and people who want to have their name attached to a cool domain. Because of this, users are subject to the despotism of a single person, which is often worse than that of a big company like Twitter, and they can't migrate out.

Since servers tend to be run by amateurs, they are often abandoned. This effectively bans everybody that signed up via that server.

There are huge issues with data duplication across servers.