Bounty managers cannot guarantee that a project will succeed in the future
This is correct, but a person who becomes a manager should have the capability of screening out the good and the bad projects. I am not saying that their decision is 100% correct, it is a screening and therefore will have false positive and false negatives. Truely it depends on the need of the bounty hunter. They often are reckless in joining the campaigns without looking into the project owners and then they complain of the project being scam.
Many of these bounty hunters even done have a little level of literacy and are posting like bots being told by some superior, probably.