I am able to add my Amex Card to samsung pay, but after a couple of days it stops working.
When the card is touched on the app the screen just flashes but does not allow it to be used to pay for an item
As @Pugs1957 has already mentioned (Thanks for sharing your knowledge! ) there might be some problems with certain banking apps and payment methods due to it being a Beta software.
Once a final, official version is released it will be certified by google (so it will be flagged as "safe") and that should no longer be a problem.