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.
Personally I don't want to guilt trip my readers into donating. There are affiliate links on the website. It's a business and it works well enough to support me. It feels like double dipping.
Above all, There's a point where it operates exactly like an ad and annoys people just like one. At the moment, American tipping culture is taking over in Germany and it feels like another instance of that.
The nice middle ground would be to ask after doing something especially nice. "That tool sure saved you a lot of time. Donate?" I think that the link in my email signature works especially well because of that.
I run a number of apps and services that don't make a dime. In fact, they cost me thousands. I don't mind.
Monetization is a real pain in the ass, and I stay away from it (for these services).
I have the ability to make a lot of money, if I choose, but that's not much fun. I don't really need the money, and I like having full control of everything I do.
I am regularly approached by folks with monetization ideas. Some, are quite good. Others ... not so much ...
Maybe I 'll pursue something, eventually, but I'm in no hurry.
Thanks for your service. Looks like a great site.
Thought of handing off some of those money-making opportunities to others, after you vet them beforehand?
I could use an alternative income stream and I am very tired of the employment grind. But I never networked and that has been biting me on the rear for the last 10-ish years.
Apologies if that's too direct. I am keenly feeling the lack of freedom and the lack of "frak you" money in the bank.
On the other hand, it allows me to work on what I love full time. This would not be possible if I relied on user donations or even public funding.
I offer a ton of free information online. I received raving feedback from people, including some that said I saved them thousands of euros. Just this week readers jokingly suggested giving me a Nobel prize or a statue. Still, donations are one percent of my revenue on a good month. They would not cover my groceries.
This is fine for me, but it's always on the back of my mind when people suggest donations as a funding method.