My favorite fraud using WebMoney was an attack on the users masquerading as an attack on the system:
A guy was selling (for webmoney) a program that supposedly cracked the system. It generated account numbers and corresponsing keys using a convoluted algorithm, and checked whether there's money in there. Sometimes, it succeeded. Some people got a lot more money, than what they invested. Those that didn't kept quiet. In reality, it was just gambling with very bad odds and a huge cut: the guy selling the program was using part of the earnings to place some money where the program found it.
As this scam was seriously damaging WebMoney's reputation, they fought it long and hard.

Ponzi schemes are done by governments as well: it's called mandatory pension insurance, with pension funds given government guarantees in exchange for keeping a huge chunk of their money in government bonds.

yes, webmoney is reliable !