alt Hacker NewsThere must be artists working on an hourly contract rate.
Maybe art is special, but there are other professions where someone can invest heaps of time and effort without delivering the expected result. A trial attorney, treasure hunter, oil prospector, app developer. All require payment for hours of service, regardless of outcome.
It'll mostly depend on the contract you sign with these services and the state you live in.
When it comes to work that requires craftmanship it's pretty common to be able to not pay them if they do a poor job. It may cost you more than you paid them to fix their mistake, but you can generally reclaim your money you paid them if the work they did was egregiously poor.