Most instances will federate with a pretty wide variety of other instances. There are a few big instances to watch out for, but for the most part the instance choice is primarily about which admin team you prefer. As long as your home instance federates with a server, you’ll be able to see it.

In particular, watch out for Hexbear and the .ml instances. Those will be some of the more extreme and/or brigade-y. There have historically been issues with brigading from some of the more extreme instances. Even calling them extreme is a sort of contentious statement, and I’ll likely get a few “how dare you, fascist, tankie, TERF, etc” types of comments if they stumble across it.

Aside from brigading, the most common reasons for defederation are related to legality (like maybe a certain instance allows porn that would be blocked in your home instance,) or unmoderated trolling. Trolling from new instances used to be a big problem, where an admin would make a server as a side project and leave it to stew. Trolls would make bot accounts on that fledgling instance, and use them to troll other instances. And since that small instance’s admin is just doing it as a side project, they’re not keeping active tabs on anything and the troll accounts are able to run rampant. Bans didn’t federate by default, so every single instance would need to individually ban the trolls.

Also, one particular quirk of federation is that federated instances can act as a proxy for defederated instances. For instance, you’re on feddit.uk, which defederates from Threads. So you won’t see Threads content normally. But maybe another instance does allow Threads, and feddit.uk is federated with them. If a Threads user posts on that second instance, you’ll also be able to see it. Because even though your home instance blocks Threads, that second instance allows it and you see it by proxy.