I find the challenges with payment and fraud to be especially interesting. Your app can have some errors but nothing threatens the future of your business more

I find the challenges with payment and fraud to be especially interesting. Your app can have some errors but nothing threatens the future of your business more than not dealing with fraud or not doing payments in the safest (perfect) way. It seems that no other feature has that. Payments/Fraud are two issues that PayPal had to overcome before succeeding and put Square into a big problem earlier this year.