Before Reddit changed API access I built an iOS app called Pager ( https://pager.app ) that allowed users to set up alerts for content posted on Reddit. It had a lot of success but the issues you highlighted here kept me from monitizing the project. Users became so demanding and I felt like if I began to take money from them it would only get worse. Looking back on it I'm not sure it was the best choice, but at least…
The usual suggestion, often given by HN's patio11, is to charge, and charge more. For some reason free customers are the most demanding, and the more you charge the more people self-select out of the customer base.
Ever try giving something away for free on an online marketplace?
If not, don't. Always charge something. Even if you'll just tell them afterwards you don't actually want/need the money.
You'll run into the worst people imaginable on the internet.