While it would be appealing to receive some of the hot new tokens for free, the truth is, the airdrops for those are long past. Assets like OmiseGo, Stellar, BAT, Golem sometimes see announcements for fake airdrops. Usually, those are attempts to steal wallet private keys. So if you see an invitation for an airdrop of an already widely traded token, best check with the official social media of the project.
Some airdrops require the sharing of a few online identities plus a blockchain address. In theory, the address is forever linked with a person. Sending and receiving from that address also taints other addresses. The small reward of an airdrop may have higher costs along the way.
For some airdrops, the coins never arrive. Or, they are a token that has no value and no way of trading. While in the past ICOs have given away valuable assets, now some projects don't even bother to send the tokens. So filling out all the forms may be a waste of time. For some projects, the referral program is also fake, further wasting time.