The main use cases are for people who are Facebook friends. I really can't fault Venmo too much here because they only deal w/ money. To me, they technically did their job. They protected the people that send the money, not the people who receive.

If you use this platform for potentially sketchy transactions, I'd say it's on the user to protect themselves. When I sell anything on Craigslist, it's always cash only. Even then, there are no guarantees.