I feel there will at some point be a "this is why we can't have nice things" moment with ActivityPub and Federation in general.
Karma is probably pretty easy to farm using fake home servers or botted accounts, and other kinds spam is probably going to be an issue if this platform reaches any level of mainstream popularity.
I think many parallels can be drawn between ActivityPub and E-mail, here. E-mail works, but not without a lot of gatekeeping, blocking and spam. Its really hard not to get blocked as a self hosted email server today, you are probably going to be mostly blocked by default until you build somewhat of a reputation for your server, etc. I foresee similar levels of maintenance being needed in the future in order to keep servers federated.
As far as moving your account, some things are easier than others to deal with. Things such as subscriptions and likes is probably a lot easier to move to a new account than entire post histories and such.