Yeah, man! Trust only reputed managers and good companies.
Yep, you're right. But unfortunately, even the best reputation managers can't give the guarantee against the payment from associated bounty projects. We must understand that it is a risk of the job.
Do the research yourself and only put in work for campaigns that look legitimate.
Many people say to do their own research. But in the fact, doing the research of legitimate or scam projects is very hard.