I've done a number of charge backs over the years, and they most often are painless. I did have one not go my way though. A hotel I stayed at charged my credit card $100 for parking. I had taken a taxi to get to the hotel. They refused to listen to me, so I did a charge back. They sent my bank a copy of a parking receipt that had no identifying information on it, just basically said, "overnight parking, $100". My bank said that was good enough, and reversed the charge back, no appeal allowed.

But at least the other time a merchant pushed back on a charge back, it worked out better. A South American airline charged me for a flight between two cities in a continent I had never even been to in my life. They used as proof a record that simply showed the flight did actually fly. My bank sided with me.