There is a simple solution to remove farms (or atleast majority of them)- introduce Jr Member as minimum requirement but that won't happen of course due before mentioned reason.
You're right most Altcoin bounty managers don't think much about the account farms in their campaigns. but of course, some still care.
I agree with the minimum JR Member rules that the bounty manager may adopt. or at least for some campaigns like Signature, Article, or Video. while Newbie accounts can be on social media with a minimum of activities that have been achieved.
the problem is, is it possible that the bounty manager is willing to apply such rules? at the risk of client satisfaction that may be disrupted.
Honestly it doesn't matter if the rank rule is applied or not. Merit can be bought. So, managers apply the jr member rule and then there will be 500 new jr members the next week. Even if they don't buy the merit, users can start meriting their alts. They may even make a post that gets them 1 merit.
A solution has to go further then rank.