My experience with pro-bono work is that more people contact you for pro-bono work after you've completed such a project and often pro-bono work is not appreciated, people looking for pro-bono work often don't know what they want done and end up eating more time than a paid customer would have taken.
As a result, I'll help with something small if asked, otherwise, any big project I charge full price. Startups where I've worked pro-bono in exchange for future payments / shares never took off compared to startups where I charged huge sums and the founders had to get of their arses to make it work or be out of pocket.
Pro-bono work also places you in an awkward position, if you start out helping with little things and the little things become more and more, when do you start charging for it? Chances are, when you start charging them out of the blue without a clear agreement upfront on when pro-bono work becomes billable, expect some angry people.
The "client" is also placed in an awkward position, what can he reasonably expect from pro-bono work, when can he reasonably expect it, etc and since he's not going to ask for those things, you'll constantly not get honest feedback, if the "client" is unhappy, he probably won't say it in fear of losing the free work being done and you will never truly know if the client is satisfied with the work you've done - which means if you're doing it for a reference, you'll never know if you're going to get an honest reference or not.
The one pro-bono project I'm currently busy with has eaten two years of my life, but unlike the other projects, I own the software, have 50% shares in the project and if the partners do not perform, I can take the software to somebody else or even drive sales myself whereas previous pro-bono projects, I did not own the patents, trademarks, or anything which left me without much negotiation power, even though I was writing the software.
On the bright side, some of these pro-bono projects exposed me to new environments, new languages, new frameworks and made me a much better developer - example, I would never have gotten into Java development (as a PHP / perl developer) or even Kotlin development were it not for one pro-bono-startup project where I was forced to work with Java.