US airlines take NEXUS. If they didnt then they'd be on hook for breach of contract as it's a valid travel document. Now unlike AC where one can select Nexus card instead of passport via OLCI, US airlines only provide a passport option for non-agent checkin. I normally just let system use my verified passport to do OLCI. I then present Nexus card to agent. Most nowadays know Nexus is valid. For the few that dont a call to their support line, or a red coat clears it up. Biggest issue nowadays seems to be agents getting confused thinking card is GE. They're exactly same format/pictures/colors/layout, just one says Nexus and one says Global Entey.