N26 is regulated by BaFin and has a full German banking license; it's just as secure as your Deutsche Bank or Sparkasse account. They also must follow the same data privacy and security laws as those banks.
N26 spared no effort to ignore and avoid any interference and control by BaFin. They are a different beast than Sparkasse or Deutsche. At one point, the only way for BaFin to contact N26 was via the customer hotline. There was no way for the banking regulation and control entity to contact them. N26 evaded BaFin as long as they coud. They've built their company and infrastructure extremely fast and extremely recklessly.
https://www.reuters.com/article/us-n26-money-laudnering-idUS...https://fintechmagazine.com/banking/n26-customer-growth-limi...