It can be hard to explain the appeal of challenger banks like N26, Revolut or Starling. Superficially, they’re pretty close to the incumbent banks. Lining up their features alongside one another, there’s not a lot between them. What they all do have is a je ne sais quoi. The apps are pleasant to use. They’re fast and well organised and smooth as butter. When it comes down to it a normal debit card does the same stuff. But the incumbent banks’ cards have hidden fees, and their apps are slow and frustrating. There are four passwords to remember. The challenger…