Contactless debit cards from a bunch of countries aren't accepted on london underground. That makes it less good for visitors. They don't even publish a list - just some will refuse to work, and you have no way to know as a visitor till you try it.

For even more confusion, some contactless debit cards won't work the first time, but if you try them 5 mins later (presumably after the main office has done a test charge on the card), it then works fine.