These issues are country-specific. Attributing Amex Germany to Amex UK or Amex US is a bad idea.

The decision to challenge a chargeback is, in the first instance, made by the vendor's merchant acquirer. In countries with tough consumer protection rules for financial institutions, merchant acquirers do not bother to challenge even if the vendor itself wishes to.

In all instances, properly documenting your chargeback at the time you make it, may take a bit more work, but can significantly increase your likelihood of success. Make certain that you include:
1. A reference to the specific provision of the contract which affords a refund as well as any applicable rules such as US 14 CFR (US) or EC 261/2004 (EU).
2. Copy of e-ticket receipt
3. Notice of cancellation or other demonstration that the carrier, not you, cancelled.
4. Your request for a refund
5. Response, if any, from the carrier denying the refund, promising the refund but not making it.

What you submit goes from your card issuer to the merchant acquirer. If your package is complete, it makes it much harder for the carrier to assert that you agreed to a voucher or that it is not required to make a refund.