AmEx with US cards in Apple Pay still results in "card not accepted" on the Underground, unfortunately--despite upgrading to the iOS 11.2 beta and re-adding. The physical contactless cards work, however. (Interestingly, they do a $1 test charge instead of £1 soon after tapping in for the first time.)
Also, adding a US AmEx card to one's TfL account results in a SafeKey box appearing. While it behaved like Chase's in that it auto-redirected back, I thought there was no support for SafeKey at all for the US.