Your login is unique to the home instance. In effect your username is the full vestmoria@linux.community but the @ portion gets added in when you log in on the linux.community page. There is some theoretical talk of federated identity that would let you use credentials across instances but the logistics of it seems daunting.
You can still read and post to wherever from your home instance, so long as the other server wasn't defederated for some reason.
The biggest issue is the risk that your instance may die. From what I have seen, most instances have given users some warning. But yeah, a few have just disappeared, leaving their users orphaned with no history. This keeps users feeling they need to stay on the biggest instances, putting pressure on them.
A method for allowing users to export/backup their ID and even import it to another instance would be ideal. We (admins) need a way to ensure it is the same user, a unique identifier. Otherwise, you'll get bots importing massive comment histories onto their accounts and faking legitimacy.
I also see this as a way for mods to maintain a ban, so a user doesn't just migrate their account to another username/instance and continue to spam or abuse readers.