Depends on the rules and person who is managing campaign, sometimes you can talk to bounty manager and leave campaign and you will receive reward and sometimes they will remove all your reward if you do so. It shouldn't get you disqualified but if it does there is nothing you can do about it. Better approach is to join campaign at late stage, few weeks before it ends, don't join campaigns if it will last for several months and if you are not one hundred percent sure you will get tokens.
Bounty hunters who have been jumping from one bounty to another bounty thinks of it as unfair rule.
If you work for a month and then you quit your job, would you consider it fair if your employer does not pay you for work you have done that month?