I think, before joining a bounty, you have to research bounty manager well. You should read the texts in bitcointalk  about the bounty manager. And especially if you sign up for a signature campaign, you should also research good the project.

You have to choose the bounty that has a good and reliable manager. If you are feeling depressed that your efforts were wasted that's the other side of bounties.
If they aren't able to reach their soft/hard cap they will never pay your efforts so if you want to get paid with the bounty you are joining. Just choose the bounty that has an experienced bounty manager and assures the participants to get paid.