that really depends on what bounty you are signing up for. not all of them are same. Some of them turn out to be a scam while others are legit that distribute on time and consequently their project does well after launch. I would say most bounties distribute except may be handful that get mixed with them.
Just look at this thread most people agree that 90% are scams and I think they are run by the same people most of the time. We need to stop this from happening.