Like with everything else, there is a learning curve. I have started by doing a lot of bounties and then, when I found out that most of them were scam/fail, I had to adapt my strategy and I've started to be very picky with my choices...
There are some features that are always prefered, independently of the project, like projects with no ICO, monthly payment, already on echanges... and then you search their use case, product, team, social channels (there are some that launch bounties and then don't update their social channels.
Lately I am very happy with bounty from Veil project, Xtrabytes and wibson
cheers and good luck