I have read story of bitcoin signature scamming their participants too (project not paying their participants for promoting them). No type of campaign is safe from scam therefore you just have to be conscious when joining a campaign. Be very vigilant when joining a campaign, note that there isn't any guarantee to get paid so don't invest all your mind and body into a project.
Hence why it's frequently suggested to only join signature campaigns that already gained a reputation here on Bitcointalk. And yes, everyone should be skeptical when joining campaigns, but with bounty campaigns though? It's almost next to guaranteed for bounty campaigns to be either a huge waste of time due to the coins/tokens being worth next to nothing, or the campaign being a scam.