@truh That goes into another point I originally talked about Discord alternatives need: the ease of creating and managing a community. Having independent servers without federation means every community needs someone with the technical knowledge to run and maintain such a server. People who (planned to) run a forum often mention this as a pain point. So federation would be better in the sense that running nodes can be left to people with the necessary knowledge.
Choosing an instance is a more significant choice than joining an invite-only Discord server.
If you used Discord before, you already have an account with all of your contacts, message histories and other communities.
When choosing an instance, you're in the process of creating an account on that instance. If it turns out your choice was bad (because you can't talk with some friends because they're on "wrong" instances), you have to find another one and start from scratch. Account migration is lossy, but even if it weren't, this alone means it's a worse experience compared to invite-only Discord servers. (edit: that is, if the federation works like here.)