Investing.com - Cryptocurrencies are unreliable and thus hard to trust, according to the annual report of the Bank of International Settlements.The report says the problem begins with the fact that cryptocurrencies are not scalable. So, as more people use the decentralized networks of cryptocurrencies, the more congested them become. The bigger the currency, the greater the potential for problems.In the worst case scenario, cryptocurrency networks can stop functioning, resulting in a complete loss of value. And all of that leads to a breakdown in trust and efficiency. The BIS, which serves as an umbrella group for the world's central banks, has been critical of cryptocurrencies before, calling them a combination of a bubble and a Ponzi scheme.