These are great lessons from an engineering perspective. From a product perspective, I've learned 3 hard lessons:

1. Fraudsters will find your product and abuse it before real users do. If you see a sudden spike in usage, it's probably fraud.

2. Real users are very emotional when it comes to their money, even if it's a "low volume transaction" like $20. Anything you build that introduces friction between them and their money will get your support team yelled at.

3. Regulators and banks are suspicious of you, for lessons they have learned from 100+ years of balancing lessons 1 & 2 above.

Balancing 1, 2 & 3 is an art and the difference between building a successful product and failure.

All the cute UI and funnel analysis you do breaks when it hits fraud, user emotions and regulation.