It's better to just find your taste, not all bounties will be able to use limited participants or fixed tokens, all you can do if find what suits you, some bounty managers can be so stubborn that your reasons won't be good enough for them, in this case nothing can change it
If you strictly adhere to the rules, then I don't think that the bounty campaign manager can deny you the reward using some flimsy excuses. But in some cases, they may change the rules when the campaign is about to end. Such scams happen all the time with bounties. In such cases, you can protest against it. If they refuse to roll back the changes, then you need to go for arbitration.