My first open source project was a failure; I worked on it for 3 years for maybe 20 hours per week. At around the same time, a number of hot SV startups started building very similar products 'as a service' - I felt that I could not compete with that kind of VC money so I gave up (I felt that open source did not stand a chance in that industry against the 'as-a-service' model). It was my first project so I wasn't too disappointed; it was mostly a learning experience. It was good to know that I could at least come up with the right ideas but that I had to improve execution. Also, picking an area/industry where open source software can thrive is important. Open source is not easy; it still has to compete with big advertising dollars and VC money.
I've been working on my second open source project for almost 4 years now. It's quite well known in its area (it got traction pretty soon after launch and kept growing steadily).
Looking back, it does feel like I could have spent my time on more lucrative things; but the problem with money is that everyone wants it; so things get very competitive and marketing-oriented. So maybe from a career/networking point of view, my open source work wasn't a bad ROI after all... For a side hustle.