The problem I have with puzzles like this is the ease with which this can be cheated. Let's take an example, someone creates a Puzzle like this and then they share the Private key with a friend (accomplice) and after a few hours or days the friend post the result and they claim the prize. The friend then gives the money back to the creator of the puzzle and they rinse and repeat the process.

What do they get out of this? Well, with a increase in the views and subscribers, the Youtube channel owner gets income and he could share that income with his friend.  

How do we validate that the winner is not a friend or family member? Well, it's basically impossible and that is the main problem I have with these puzzles.