Even if you are thorough with your researches, you might bump into a scam project and waste your time with it without getting any reward.
If you want to be slightly safer, just choose a bounty which pays BTC or any exchangeable currency instead of waiting for a payment in their native currency, which would be a bag of trash!
Due to seeing failed or scam bounty campaigns I now prefer to join in the signature campaign who pays in bitcoin so I can sure I earn profit in all my works. But, joining in a signature campaign who pays bitcoin is quite hard especially if they required a good quality post and the participants that they want is the member who gains merits in the last 120 days. It's now hard to determine what are the good projects and I agree with you, even you do your effort doing research in a good project sometimes it still ends up in a scam.