Indeed late bounties can still pay rewards, but based on my experience, there are many bounty campaigns haven't paid their participants.
The most important thing from bounty campaign is first we must know 100% that they will pay us and when they will pay.
without that, we should avoid this kind of bounty campaign.
I agree with you that it's a rare thing that late bounties will pay. However, I don't understand why you say we must know 100% they will pay us. There's no way for us to make sure that they will pay the reward 100%. That's one of the characteristics of doing bounty hunting, there is always a risk even a small rate. Therefore, the best we can do is to do our researches to reduce the risk of not getting paid, we can't guarantee anything!