I think they aim to expand to the US. I'm an N26 customer and US citizen and they asked me to chat with them (paid) about whether I'd use them in the US, etc. (I quite like them, tbf)
I don't want something with a high interest rate, or stock picker, just a way of putting a firewall between the cards and my cash.
Living and working in Asia for many years, I always sympathized with the difficulties my American expat friends had in trying to open any sort of local bank account at all, even a current account. Most banks say no the minute they hear you're a US citizen.
Not being able to open a bank account in the country in which you live has serious knock-on effects in terms of getting paid, or renting a home. I am eternally amazed that American citizens, who supposedly value personal freedom, put up with having one of the world's most aggressive tax regimes.