The main reason for bounty hunters not getting paid is a failed project and a scam project, a failed project is when the developer cannot reach the softcap and they decides to return the investor's funds, while the scam project is when ICO developer runaway with the funds from investors.
based on my experience most ICOs pay their bounty hunters even though the paid tokens are not worth at all, , I don't know such ICOs are categorized as scams or not