The Easy Bank Transfers thread, might be worth a read.
Short version:
- Monzo now has its own open banking platform.
- It currently only does bank transfers (in labs) but more features coming
- Starling isn’t supported because their API is different to everyone else’s
- Amex also uses a non-standard API.
My inference is therefore that you won’t get Amex support from the get go when this comes in house. And, as others have said, this may well become a paid feature. (Personally, I’m not sure that the balance is paying for, but the native Monzo experience with transactions etc might be).