Someone asked about Amex in the other thread.
As the Open Banking team said in the Easy Bank Transfers thread, Amex don’t use standard Open Banking APIs, so as Peter guessed a few days ago, it will take a while longer before Amex is supported 
At launch we support adding payment accounts and credit cards from Lloyds, Barclays, Barclaycard, HSBC, NatWest, Santander, Halifax, Nationwide, RBS, Bank of Scotland, First Direct, Capital One, MBNA. We estimate this covers over 85% of Monzo customer’s other accounts.
Not all providers share all kinds of savings accounts with us; for example, ISAs are not usually included.