For the first, you're always at the mercy of the whims of the mods for whatever service you're on. The difference with federated services is that you can move to or even set up a server with different mods. Can't do that with Twitter.

For the second, that's a limitation of Twitter. If you had 10,000 followers on a federated system, you could switch networks and your old followers could just re-follow you there. They wouldn't have to switch with you.