I don't suggest to make only Airdrop or Signature Champion as the main earning source because signature campaign is not forever or anytime a signature campaign can end and face earning problem later just like people do with airdrop.
I don't think a signature campaign is as unreliable as an airdrop though. The competition and time you need to invest are significantly harder, but you'll likely earn more for each week you participate in a campaign compared to airdrops. Even if a campaign ends within a week, there are chances for you to apply to another one quickly compared to airdrop. I think bounty is the one that is very similar to airdrop.
I do agree that relying on a signature campaign as your main income is risky, but it's still a decent shot to get crypto if you don't want to spam faucets or spend money to buy one.
It'll be completely absurd to compare signature campaigns to airdrops, I mean who'll even make sure a comparison, maybe back then I'll agree that airdrops can actually be as profitable as signature campaigns, but presently, airdrops are nothing more but a total waste of time, there are no assurances of making anything meaningful from airdrops, you can participate in tons of airdrops and still end up filling your bag with valueless tokens and wait all your life for at least one token to become valuable, but at the end, it's still a total waste of time.
Unlike the signature campaigns, at least you're sure that if you're accepted in a campaign that you'll get paid after a week, except there's a problem with the campaign or the manager, but regardless, signature campaigns offers you more assurances than the airdrops.