In Argentina, the main problem for Uber is that the court ordered the credit card companies to block payments made by local clients to Uber, because of the company not adjusting to local regulations. So, if you don't have a credit card from another country, you can't use the app, and that makes it hard if not impossible for them to build the necessary economy of scale that their kind of operation needs.

Yup, that's also an issue. Mastercard does work though, but considering how few people have credit cards (and even fewer have Mastercard specifically), they're having problems hitting critical mass.