It might fail, but it probably won't. Also your card might pick up static and not interface with the terminal. Or you might break it by bending it in your pocket (happened to me a while back). You say there's no UI for cards, but that's only the case for transactions under €20. With phone, no PIN required for transactions over X dollars. One motion for everything.

Then add that you don't need to carry a wallet if you have phone payment. Throw your phone in a sleeve (or use a watch!) and go running, stop for your smoothie on the way home.

Finally, since it's a phone I can call it when it's missing or my watch will tell me if it's in the Uber I just got out of. Even works when intoxicated!