Only certain FOSS is worth working on for me. Here's my heuristic as of today. * Don't work on FOSS that supports a SaaS or proprietary product. You're enriching someone else's ecosystem for no money. You take on the communication bandwidth and issue load of dealing with not only your issues but their issues too. Your software is useless if theirs fails. It's thankless. * Don't work on FOSS you aren't using yourself…
It also means you’re very restricted in license choices since you otherwise never know if your project might get picked up in the future and used in a proprietary service. This likely means you have to use something like AGPL.