Flights on U.S. carriers that originate in the EU fall under the EU rules for passenger compensation/protection. US (and presumably others) claim that mechanical irregularities are out of the carrier's control and hence don't qualify for the EU's passenger compensation/protection rules. They ignore the fact that mechanical irregularities are considered within the carrier's control domestically.

US' CofC does have language referring to the EU rules, but if a passenger accepts whatever the airline offers the passenger gives up any claim under the EU rules.

I think each country in the EU has a department that investigates delays/cancellations and determines what compensation a given passenger is entitled to under the EU rules. That would probably be the best avenue for resolving a claim although it would probably take a while to get resolution.

Jim