I created a shortener (for personal control, tracking, etc) and can really, really identify with #3 and #4. I was blocking loads of nonsensical domains each day for weeks before the hosting company pulled the pin and I just gave up and pointed my domain to bit.ly.
Yes, that's true and I should've done that but I wanted to keep the service running quickly through that domain and bit.ly pro did that very easily, it "just" meant losing historical links.
Easy to make the wrong decision when you, well-intentioned, set something up with no view to profiting from your time and then get slammed by spammers mercilessly, get no love from your host, etc.
I don't know that I'd be the only one who loses a fair swag of their time and money (work, side projects, etc) either dealing with spam or trying to protect against it. So frustrating.
Note that this in a bad idea in general, not just for URL shorteners but also for your personal website and even more for a project or company webseite.
There is a classic article by Tim Berners-Lee on that topic, written in 1998:
