Unidan could just create an account on multiple instances and vote for his posts/comments with all these accounts. That way his content would gain more attention than those of sincere users.
In case of malicious bots (like those annoying bootleg bots on reddit), it might even be profitable for them to create their own instance(s) just for that purpose.
Is there a mechanism to prevent that? (other than user/instance banning and the introduction question on user creation)
Huh - I had not even considered downvotes in other instances… Yeah that seems problematic. Maybe rather than defederating downvotes, there could be a way to differentiate between local and global up/downvotes.