In the five years that I used them (maybe 4 rentals a month), I only had one issue. I rented a Toyota Matrix that wasn't there. They gave me the BMW 3 series beside it for the same price.
On my end, I had two incidents. I was late once because an accident essentially shut down the beltway. They made arrangements for the person after me, and gave me a pass. The other time, a regular car was parked in the spot and all of the other spots on the street were filled. The closest spot was three blocks away, so they had me take pics of the offending car and send them as proof.