To add data point on AMEX. The charge back feature is among the reasons why AMEX is not as acceptable by merchants - the high merchant fee and the charge back is too much work for the merchants to object. So many merchants eat the non-legitimated charge backs and finally decided to drop AMEX from their acceptance. This is not to say about legitimate charge back such as the examples cited by the poster.

Definitely not all AMEX charge back succeeded. There are reported cases in various forums on FT when an AMEX charge was initially reversed, it came back when the merchant did contest and finally AMEX cited with the merchant. A few of those involved airline tickets when the cardholders were forced to buy an expensive ticket to fly home when their original tickets did not work for whatever reasons. The cardholders instead of resolving their problems with the issuer of the original tickets, they mistakenly disputed the charges on the new tickets which the airlines did fly them home. Inevitably those charges were put back to their cards because the merchants in this case, did fulfill their obligation by providing flights to those ticket purchasers. You read these type of stories every few months in various airlines forums.

In one of the cases I read, the cardholder was actually advised by AMEX NOT to file dispute on his one-way ticket bought to fly LAN when his AA award ticket did not work, because there is no way AMEX would proceed the dispute when LAN did fly him home. After months of working with AA, the airline finally offered him compensation options, including one was a check to cover his LAN flight, the other one was to issue an equivalent AA credit AND some decent miles. He chose the check. In that long thread, there were many ill-advices about to file a dispute on the LAN charge, until the OP finally came back to reveal this was not even an option because AMEX "advised" him NOT to go that route.