The timing of this as the switchover of Costco cards from Amex to Citi is happening certainly suggests a scaling problem regardless of the "updating its systems" excuse.
I've heard Amex blamed for losing the Costco deal, but given Amex's superlative customer service and Citi's less-than-optimal reputation, maybe Costco will turn out to be the bigger loser.
I've heard Amex blamed for losing the Costco deal, but given Amex's superlative customer service and Citi's less-than-optimal reputation, maybe Costco will turn out to be the bigger loser.