a) you should not be the owner (to avoid pet projects that are not actually useful) of the project or at least not the sole owner
b) ideally it should be some high impact projects that have little to no corpo sponsors as opposed to something like React
c) if your contribution is not merged in, it should not count as "work done"
The Hacktoberfest incident is a good example: The program offered a T-shirt to people who had a PR accepted. The result was tens of thousands of useless PRs across open source repos and maintainers begging for the program to stop so they could stop dealing with useless PRs. https://joel.net/how-one-guy-ruined-hacktoberfest2020-drama
In a situation like this you can’t assume that the set of people and the type of work being submitted will remain the same as before the incentive appears.