Nah, bots can farm reputation better than humans can.

The problem is that federation assumes good intent of the admins of other instances. If someone builds a network of instances intended to manipulate votes et cetera, I can’t see how the fediverse could resist that.

Instance A can’t trust that votes from instance B are legit. As soon as there’s no trust then federating even with an aparently trustworthy instance becomes too costly in terms of time and effort.