There are many parameters, first of all look at the development team and the budget of the bounty. Also, activity in social networks is important. when choosing a bounty.
True, for myself in choosing a bounty is if they have a product, a dedicated team and I always go to their website to learn a lot for their project and I make sure to read their whitepaper for more information. So that's when I will join their bounty program. Sometimes even a nice whitepaper, a good team and others fail the project.