For Airdrops they are not 100% trusted, sometime newbie people also start airdrop they think that they may get some Donation or some early token buyer. To run away.
But some projects are trusted but they are not able to get listed on exchanges because of insufficient money.
Bounty campaigns also has same story.
You can join in any or in all Airdrops, because it doesn't require your attention daily. You just need to fill the form once (many cases).
You can find list of airdrops here manage by me http://cryptocreed.com/airdrop/