It's pretty much the same as with email. If you create an account at Mastodon, you are user@mastodon. If Mastodon goes down, that account is gone. If Mastodon gets sketchy, spams, posts NSFWs, etc. it might get blocked by other hosts and your account won't be able to participate on other server.

If you just use throwaway accounts, it doesn't matter, just create one on each host. If you want a permanent account, either create it on one of the bigger servers or in a community you trust and expect to survive for a long time.

Accounts also only work on the server they were created. While you can post with your feddit account on lemmy.world, you have to do so on feddit, you can't login on lemmy.world with your feddit account.

The idea of making accounts transferable has been around, but so far isn't possible:

• https://github.com/LemmyNet/lemmy/issues/506
• https://github.com/LemmyNet/lemmy/issues/1985

I hope that the whole Fediverse can switch to crypto identities (as in GnuPG, no need for blockchain) sooner or later, it would make this whole thing way more robust and easier to truly decentralize. What we have right now is not much different from lots of individual websites with a little bit interconnection for easier discovery.