Honestly speaking you can't really tell which project is credible right from it's early stage and if they come with the notification that people would have to pay certain amount of money as a part of the entire process then a lot of persons won't care more and that will mean they won't have a fledging community enough to drive the value of the token high.
So they do keep the part of transaction requirement towards the end of the project to be airdropped so that persons that have spent time and data following up the project would be compelled to have no alternative than to oblige. All of these are predetermined, and that a project looks credible throughout the time doesn't always guarantee that it will pay in the long-run.
Yeah, too bad we can't say for sure before it's too late.
That's the problem of it, and it's the risk everybody takes or not, their choice.
You either press the pedal full-on or never start the road in the first place. That's how I see it, because if you already got your funds there, you believe in it - and you won't stop probably, because your investment is at stake. It would be really hard not to.