I'm the sole maintainer of a very small open source project at NASA. Despite the small codebase, I think I am still in the top ten most starred repos on github.com/nasa.

By far the biggest problem is that we don't get any time at all to devote to maintenance. The second the NASA project I wrote the software for stopped paying for my time, the project became a personal labor of love/hate. If I ever leave NASA, I'll lose my ability to merge PRs and presumably the project will be completely abandoned despite receiving fairly regular contributions from the community. I have no particular use for the software myself, so I sporadically review and merge PRs in my spare time entirely out of a dubiously founded sense of obligation to the people who bother to submit them.