In my (limited) experience they mostly don't work because the people/projects that need donations are way too coy about asking for money. Donation buttons get tucked out of sight and/or cloaked in twee language like "buy me a coffee!"
I like to point to https://archiveofourown.org as a counterexample. Hardly the most critical project in the world - it's just fanfiction - and yet they raise a few hundred thousand dollars a year in public donations (solidly surpassing their budget goals) because they run an actual, clear, focused fundraising campaign twice a year.
